QAMISHLI, Syria -- Kurdish fighter Seewar Sofi still wears his uniform. It matches those of his comrades in the photo taped above his hospital bed. The fatigues he was issued as a member of the YPG, or the People's Protection Units, are now folded over the stumps of his legs, both amputated above the knee. The rolled-up sleeve of his shirt reveals a right hand that resembles a claw. The 20-year-old native of Tel Tamer village was maimed last month by an Islamic State bomb on the Kurdish front lines in northern Syria. The explosion brought down the house he was entering and killed his friend Adnan. Mr. Sofi washed cars for a living before he joined the YPG last year. In his words, he knew he could be killed one day or badly injured. He has no regrets. "The YPG was made for us, the Kurdish people," he said. Mr. Sofi is one of more than 40,000 men and women fighting with the YPG, battling Islamic State forces in what has become a pivotal and bloody front in the regional war. Most are lean and motivated guerrillas who come from the three Kurdish-run enclaves in northern Syria, an area known to the Kurds as Rojava, and Kurdish neighborhoods of Aleppo. These fighters represent a powerful, pro-Western force that could be the best bet for defeating the Islamic State, also known as ISIS or ISIL, in Syria, and have scored a string of successes in recent weeks. Even so, given the ethnic, territorial and political crosscurrents that have long buffeted the region, it's a lot more complicated than just winning on the battlefield. YPG has been dogged by accusations of looting and forced displacement of Arab families, most recently in the border town of Tal Abyad. Its emergence as a serious fighting force, backed by U.S. air support, is making Turkey, a NATO ally which has long been struggling with its own sizable Kurdish minority, nervous. Any Kurdish gains are also guaranteed to unnerve leaders in Iraq and Iran as well, which also have significant Kurdish enclaves. "The U.S. is starting to put its weight behind the YPG, seeing them as an integral part of the anti-ISIL strategy," said Michael Stephens, the Qatar head of defense think tank Royal United Services Institute. In recent weeks, the YPG has driven back the Islamists, notably from Tal Abyad in the offensive that cost Mr. Sofi his limbs. What the YPG gained was international recognition and a supply corridor between the eastern region of Jazeera and Kobani, the "Kurdish Stalingrad." Victory at Tal Abyad also severed a key smuggling route for fighters and material moving across the Turkish border to the de facto Islamic State capital of Raqqa. Islamic State fighters have since struck back at the YPG. In a brutal counterstrike on Kobani, extremists slaughtered more than 200 civilians on June 25. Islamic State sleeper cells attacked inside Tal Abyad last week. Intense fighting is continuing in the large city of Hasaka to the east. So far, YPG fighters have held their ground. Airstrikes by the U.S.-led coalition have enabled many of the YPG's recent successes. Still, U.S. support to the Kurds has increasingly alarmed Turkey, which sees the YPG as terrorists. Ankara has a long-running internal dispute with the YPG's parent organization, the Kurdistan Workers' Party, or PKK. Spokesman Redur Xalil said the YPG has been fighting radical Islamist groups in Syria for three years. "We are doing more than the [Iraqi Kurdish] Peshmerga and more than the Iraqi army," said Mr. Xalil, who estimates that the YPG has taken nearly 3,500 square miles of territory from the Islamic State since April. The long-term agenda of the YPG is the subject of fierce debate. After all, it is the armed wing of the Syrian Kurdish Democratic Union, or the PYD, which wants autonomy for the Kurds within a future confederated Syria. The PYD already has led a coalition in adopting a constitution and running an administration that -- at least on paper -- advocates democracy and equal rights. "This is the kind of democracy we are working toward: Everyone will be equal in rights and in duties irrespective of religion, gender or beliefs," said Elizabeth Gawrie, a Syriac [Assyrian] Christian and the deputy head of the Jazeera enclave. As for the YPG, its primary goal is to destroy the Islamic State, not to create an independent state, Mr. Xalil said. Turkish skepticism Turkey isn't buying it. It sees its own borders under threat and worries that the Syrian Kurds will link up with the ethnic brothers across the border. "The primary concern for Turkey is that the YPG and their overlords the PYD are the sister party of the PKK," said Aaron Stein, a Turkish policy analyst and nonresident fellow with the Rafik Hariri Center for the Middle East. The PKK is listed as a terrorist organization by Ankara, the European Union and the U.S. Analysts say Turkey is worried that the YPG will push westward from Kobani, linking the Kurdish enclaves and changing the balance of power on Turkey's border. Many attribute Ankara's reluctance to join the U.S.-led coalition to defeat the Islamic State to precisely those fears. That scenario prompted Turkish President Recep Tayyip Erdogan to declare last month that he would never allow a new state on Turkey's southern border. Mr. Erdogan convened a meeting of his national security team last week to discuss the situation, sparking uproar in Turkish media about a military intervention. The YPG's relationship with the government of Syrian President Bashar Assad, battling for its very survival against the Islamic State, al Qaeda-linked forces and a U.S.-backed rebel army, is also complicated. Despite limited clashes, there is an uneasy coexistence, including a government-controlled area within the YPG city of Qamishli. "It is important for all of the world and us to fight ISIS right now, not the regime," Mr. Xalil said. Dealing with the Islamic State ahead of Mr. Assad fits Washington's priorities, Mr. Stephens said. "These are the guys who are as not as bad as the others," he said. "They are the coalition's best bet in Syria. I don't see any other choice." The conflicting agendas have proved a tough diplomatic challenge for the Obama administration, which has continued to support the YPG campaign with airstrikes while trying to ease Turkey's fears. Ambassador to Turkey John Bass on Friday told reporters in Ankara that the U.S. remains committed to a "unified Syria" and said Washington has informed YPG leaders of its concerns over events in the Kurdish-controlled regions of Syria. Whoever controls the border "must be committed to a future Syria that is unified and democratic," Mr. Bass told the Agence France-Presse news service, in what was seen as a veiled acknowledgment of Turkey's fears about the creation of an autonomous Kurdish state in Syrian territory. Some in the region say the reality on the ground has already shifted the political landscape. Given its successes against the Islamic State, "the PYD is now a force in its own right, with not only an organized and disciplined armed wing but also a viable autonomous region in a country embroiled in devastating conflict," Columbia University visiting scholar Ranj Alaaldin wrote in a commentary last week on AlJazeera.com. "Turkey has no alternative but to accept that a Syrian Kurdistan has become more sovereign and more powerful." Syrian Kurdish authorities have denied accusations of YPG rights violations, ethnic cleansing and atrocities against Arabs. "Any violations that have occurred were instances of personal behavior," said Akram Hesso, head of the Jazeera enclave. "We will investigate and bring to justice those who have might have done this. In times of war, violations occur, but they were not systemic. There was no political or military decisions supporting them." But military success has not solved all of the Kurds' problems in Syria. A hostile Turkey has left the YPG isolated and largely self-reliant. The areas under its control include fertile farmland, oil reserves -- sometimes refined at crude roadside stacks -- and a border with Iraqi Kurdistan. The recent opening of the corridor to Kobani enabled the transportation of much-needed military materiel, Mr. Xalil said. The group is still forced to smuggle in other essentials -- including medical supplies to treat injured fighters such as Mr. Sofi and the other 16 seriously wounded fighters now in the Qamishli military hospital. Mr. Sofi said he doesn't want pity. He does desire one thing though, more than anything else. "I just want to return to my unit," he said.

David R. Sands contributed to this report.