The US has demanded that Syria withdraw its troops from Lebanon now, saying promises are not enough.
Syrian troops were on the move in Lebanon on Monday
White House spokesman Scott McClellan called for the "complete and immediate withdrawal of all military and intelligence forces from Lebanon".
The comments came hours after the presidents of Syria and Lebanon said Syrian troops would be pulled back - but not out - by the end of March.
Bashar al-Assad and Emile Lahoud did not name a date for a full withdrawal.
They said only that Syrian forces would withdraw to the Bekaa Valley in eastern Lebanon by the end of the month.
'Not far enough'
The joint announcement followed weeks of international pressure and protests in the Lebanese capital Beirut.
But President Bush's spokesman dismissed it as "a half-measure that does not go far enough".
SYRIA IN LEBANON
Military intervention begins in 1976
30,000 troops in Lebanon during 1980s, currently 14,000
Syrian forces help end Lebanese civil war in 1990 and maintain peace
Calls for Syrian withdrawal increase in 2000 after Israeli pullout from S Lebanon
UN resolution calling for foreign forces' withdrawal in Sept 2004
"UN Security Council Resolution 1559 was very clear - all foreign forces need to leave Lebanon" immediately, Mr McClellan said.
He said the Lebanese people were demonstrating that they wanted to live free from outside interference.
French President Jacques Chirac backed the US call for implementation of the UN resolution, his office said on Monday.
Mr Bush and Mr Chirac discussed the issue by phone, Chirac spokesman Jerome Bonnafont told the French news agency AFP.
Tens of thousands of anti-Syrian protesters have been demonstrating in Beirut's Martyrs Square, waving Lebanese flags and chanting slogans calling for "freedom, sovereignty, independence".
The square has been the scene of demonstrations ever since the death of former Prime Minister Rafik Hariri three weeks ago in a car bomb that many Lebanese blamed on Syria, although Damascus has denied responsibility.
Monday's Syrian-Lebanese statement said up to 5,000 Syrian troops would leave outposts overlooking Beirut, but the fate of the 14,000-strong Syrian garrison has been deferred.
The "size and length of stay of Syrian forces in Bekaa" would be agreed a month after the initial redeployment.
After that, there would be further negotiations on a complete withdrawal.
The BBC's Kim Ghattas in Syria's capital, Damascus, says the statement was not specific on whether the announcement meant a final pullout of all Syrian forces.
Syrian officials have worded their statements to leave room for their troops to remain just across the border in Lebanese territory, she says.
Some Syrian soldiers in Lebanon were seen packing, but no troop movements have been reported yet, our correspondent adds.
Syrian troops entered Lebanon in 1976 as peacekeepers during the 1975-1990 civil war, and have remained since, while Damascus has dominated Lebanese affairs.
The Syrian president said that after the redeployment, Lebanon and Syria "will have fulfilled our obligations under the Taif accord and under [UN Security Council] Resolution 1559".
The 1989 Taif accord, which ended the Lebanese civil war, stipulates a phased withdrawal, while the 2004 UN resolution calls for foreign forces to leave Lebanon and its militant groups to disarm.
Lebanon's pro-Syrian Hezbollah guerrilla movement has denounced what it sees as Western interference and called for a "massive popular gathering" in support of Syria on Tuesday.
Germany, Russia and Saudi Arabia have also called for a withdrawal of Syrian troops from Lebanon.