WASHINGTON (Reuters) -- U.S. President George W. Bush lobbied for international pressure on Syria to immediately pull its forces from Lebanon after his administration dismissed a Syrian promise on Monday to withdraw in two stages.

"We believe it is a half-measure that does not go far enough," White House spokesman Scott McClellan said of the Syrian pledge. "We need to see Syria act," he said.

Bush telephoned French President Jacques Chirac and the two leaders agreed to keep urging Syria to conduct a "full and immediate withdrawal of foreign military and intelligence personnel" from Lebanon, White House spokesman Trent Duffy said.

"Both leaders are committed to a sovereign and independent Lebanon and they agreed to keep in close communication on the matter," Duffy added.

Bush also discussed the issue in a phone call with Crown Prince Abdullah of Saudi Arabia, which last week joined the international call for Syrian troops to leave. Bush thanked Abdullah for his "personal efforts to promote stability in Lebanon," Duffy said.

Under the withdrawal plan announced in a joint statement, the leaders of Syria and Lebanon agreed that Syrian forces would complete a pullback to eastern Lebanon by the end of March. Military authorities of the two countries would then decide how long the forces should remain.

Pressure has mounted on Syria for a troop pullout following the assassination of former Lebanese Prime Minister Rafik al-Hariri last month. Syria has denied any role in that killing.

The plan announced on Monday did not set a timetable for a full withdrawal. The United States has insisted that a complete pullout was essential to Lebanese elections due by May.

"This does not add up to Syria leaving Lebanon. Nobody has said all troops are leaving Lebanon," said a State Department official. "We will continue to hold their feet to the fire."

Said McClellan, "We're continuing to work with the international community ... and we're continuing to look at the way forward, if Syria does not act."

OPTIONS

Options could include seeking a new U.N. resolution that would demand withdrawal and possibly threaten international sanctions, officials and diplomats have said. No decisions have been made and the international discussions are an early stage, an administration official said.

Asked whether the United States was worried about the potential for instability in Lebanon if the Syrians pulled out, the State Department said Lebanon was "perfectly capable" of running a fully sovereign country.

"You cannot use the supposed weakness of a Lebanese government as some sort of excuse for not getting Syria's troops out," said the spokesman.

Syria's ambassador to the United States, Imad Moustapha, laid out some details for Syria's withdrawal from Lebanon in an interview with CNN.

"We were talking about less than two or three weeks for the first phase and less than two to three months for the second phase," Moustapha said.

He added: "The leadership are meeting to actually put the timetable (together). This is what we are doing. It is clear, it is categoric, there are no doubts about this."

The pullback would be the biggest single such move since Syrian forces intervened in Lebanon's civil war in 1976. It now has some 14,000 troops there, down from 40,000.