BEIRUT, Lebanon -- The Russian military is expanding its footprint in Syria, setting up operations at an airfield in a northeastern, mostly Kurdish province across the country from its main coastal base. In an adjacent province, locals say the United States is intensifying its aid to Kurdish militias, even taking over a small agricultural airport; Pentagon officials denied this. And some Syrian fighters say Russia has reached out to Sunni tribes, offering to help them fight the Islamic State extremist group in the east after similar American efforts failed.
As diplomats from Russia and the United States work to bring Syria's government and its domestic opponents to peace talks next week, the two countries are jockeying for position on the ground in Syria in a battle that will continue regardless of any peace deal: the fight against the Islamic State.
Both powers seem to be presuming that the peace effort will fail and digging in for the next phase of war. Their separate, and competing, new efforts against the Islamic State are part of a parallel battle over who will lead the fight against the extremist group, also known as ISIS and ISIL, and possibly take credit for defeating it.
Western leaders have long hoped for a day when the Syrian government, rebel fighters and their international backers would unite to defeat the Islamic State. But that possibility seems more remote than ever because of a fundamental strategic disagreement between American and Russian leaders. Russia is allied with the Syrian government of President Bashar al-Assad and views the armed insurgency as inseparable from the Islamic State; Washington considers Mr. Assad's government and its crackdown on opponents a principal reason for the rise of the Islamic State.
In Paris on Thursday, Defense Secretary Ashton B. Carter called the Russian strategy "tragically mistaken," focused largely on "supporting regime forces against opposition forces." He added, "I hope that they can be brought ahead in the right direction, in which case of course we can work with them."
A senior Pentagon official, speaking on the condition of anonymity to discuss operational matters, said there was no current military collaboration with Russia. That, he said, was a subject for diplomats, with the State Department perhaps trying to build it into an agreement with Moscow over an exit strategy for Mr. Assad.
Currently, however, the global powers seem to be competing for the same allies.
Each says it supports Kurdish and Sunni fighters in the area, even as American officials insist that there is no coordination other than deconfliction, or making sure that their aircraft do not threaten each other.
Witnesses in Syria and American officials confirmed that the Russian military had taken up positions at an air base near the northeastern city of Qamishli. One fighter said the military had recently reached out to Sunni opposition fighters from Syria's eastern Deir al-Zour province: the same groups the Pentagon recruited for its failed train-and-equip program.
The fighter, who asked to be identified only by his first name, Ahmed, to discuss operational matters and negotiations, said he had been at a recent meeting with Russians, Kurdish militias and Arab rebels in Tal Abyad, a small city near the Turkish border.
Ahmed said that his group had previously been supported by Ahmed al-Jarba -- a Syrian opposition figure closely tied to Saudi Arabia, which is staunchly opposed to Mr. Assad and Russia's role in Syria -- and that the Russians' outreach had made the fighters wonder if Saudi Arabia and Russia had a secret understanding.
"We were really surprised to meet a Russian delegation in our headquarters," he said.
Ahmed said the Russians had offered his tribe, the Shweytat, weapons and assistance to take back their home area in Deir al-Zour from the Islamic State. The Shweytat are bitter enemies of the militant group, which slaughtered hundreds of their people after they refused to submit to its rule. They have contributed to the ranks of Arab forces fighting alongside the Kurdish People's Protection Units, or Y.P.G., Ahmed said.
But the aid plan was suspended, he said, after the Russians asked him and his comrades to gather 300 fighters and they could not immediately muster more than 200. In any case, he remained skeptical that what the Russians were proposing would be enough to have an impact on the effort to take back Deir al-Zour.
"What can I say?" he said. "It's very hard to liberate Deir al-Zour with this amount of weapons."
In the northeast, in Qamishli, a Syrian activist network called the Local Coordination Committees has reported in recent days that 100 Russian military personnel have been deployed at the city's airport, and that Russian officials met with both government officials and Kurdish militia leaders to discuss deploying forces in the city, the provincial capital. These reports added to accounts that the Kurds, who have what amounts to a nonaggression pact with government forces, coordinate with both sides: American and Arab insurgents on one hand, and the government and Russia on the other. But the activists said Russians were being deployed in areas controlled by government forces, not the autonomous zones carved out by Kurds, where Americans are aiding the Kurdish and Arab militias.
Two Pentagon officials confirmed the details of the Russian deployment, including the critical point that Moscow does not seem to be focused on directly supporting fighters in the same places as the Americans.
"I'm not sure I'd characterize it as providing support to the same people as we are," Gen. Joseph F. Dunford Jr., chairman of the Joint Chiefs of Staff, said in an interview in Brussels.
Asked about reports by local activists that the United States was establishing a new base in the adjacent northeastern province of Hasaka to land Special Forces and supplies, he said, "Clearly, I won't talk about what our Special Forces are doing."
Another United States military official said, "Simply not true."
However, Yaman, an anti-government activist from Hasaka who uses only his first name for safety, said that two jets loaded with light munitions had landed recently at the Hasaka base, near the town of Ramaylan, on a small airstrip previously used for crop-dusting planes.
He said there were about 150 United States military personnel there, guarded by Kurds who prevent anyone from approaching. The same account was provided by the Local Coordination Committees and the Syrian Observatory for Human Rights, a British monitoring group with contacts on the ground.
In Washington, Pentagon and other American officials said they were monitoring the Russian deployment at the Qamishli airport but did not necessarily view it as a negative development. Some American officials warned that all parties involved -- Russians, Kurds, Sunni Arabs and government forces -- were seeking advantage from the troop movements.
"Look, the Russians are trying to play both sides of the fence, telling the Kurds they'll use the base to fight ISIL and telling the regime they'll use the base to improve their position in the country," said one senior Defense Department official, who spoke on the condition of anonymity to discuss internal American assessments. "It's also an attempt to try to set themselves up as the hero."
Another senior Pentagon official emphasized that moving 100 Russian soldiers to an airstrip in the northeast, far from coastal bases in the west where the bulk of the Russian ground forces and combat planes are based, was a relatively small deployment, more significant geographically than militarily.
It would also bolster the narrative that Russia is attacking the Islamic State as well as other Western-backed groups opposed to Mr. Assad's government, the officials said. Opponents of the Assad government have accused Russia of targeting their fighters almost exclusively, largely ignoring the Islamic State.
Russian officials also no doubt recognize that openly cooperating with the Kurdish militias, especially so close to the Turkish border and amid recent Turkish-Kurdish military clashes, would irritate the Turkish government, American officials said. Moscow has been looking for ways to taunt Turkey since Turkey shot down a Russian warplane in November.
American military officials voiced one important cautionary note. Any increased Russian aerial activity in the northeast, they said, could interfere with American Special Operations forces who recently began advising and assisting Syrian Arab fighters battling their way toward Raqqa, the Islamic State's de facto capital.
"With the entry of the guys into Syria, we have to be very careful about the airspace in and around northern Syria," said the American military official who spoke on the condition of anonymity to discuss operational matters.