US President George W Bush has demanded Syria's complete withdrawal of troops from neighbouring Lebanon by May.
Protests against Syria have been going on for weeks in Lebanon
"When we say withdraw we mean complete withdrawal - no half-hearted measures," he said, amid growing world pressure.
Syria's President Bashar al-Assad is due to announce on Saturday that some troops will be withdrawn, but some will be relocated, according to reports.
Syria has come under intense pressure to withdraw since the killing of former Lebanese Prime Minister Rafik Hariri.
Lebanese opposition groups have blamed Syria for the 14 February bomb attack - a charge Damascus has strongly denied.
In an interview with the New York Post, Mr Bush said the withdrawal must happen before Lebanese elections in May.
He said: "The subject that is most on my mind right now is getting Syria out of Lebanon, and I don't mean just the troops out of Lebanon, I mean all of them out of Lebanon, particularly the secret service out of Lebanon - the intelligence services.
SYRIA IN LEBANON
Military intervention begins in 1976
30,000 troops in Lebanon during 1980s, currently 15,000
Syrian forces crucial in ending the Lebanese civil war in 1990 and maintaining peace
Calls for departure of the Syrians increase gradually with Israeli withdrawal in 2000
UN resolution calling for withdrawal of all foreign forces passed in Sept 2004
"This is non-negotiable. It is time to get out... I think we've got a good chance to achieve that objective and to make sure that the May elections [in Lebanon] are fair. I don't think you can have fair elections with Syrian troops there."
Asked if there was a threat of military action if Syria did not comply, Mr Bush said: "My last choice is military."
France, a traditional ally and former colonial ruler of Syria, has also toughened its stance.
President Jacques Chirac called on Friday for the "full, entire and immediate" implementation of a UN resolution seeking Syrian withdrawal from Lebanon, AFP says.
Mr Assad is due to address Syria's parliament on Saturday.
He is expected to say some troops will pull back to eastern Lebanon, news agencies quote unnamed Lebanese officials as saying.
If so, he will stop short of the complete withdrawal of some 15,000 Syrian troops and thousands of Syrian intelligence personnel demanded by Mr Bush and other world leaders as well as the Lebanese opposition.
But it may help to ease pressure if it is seen as a clear step towards a full withdrawal, says the BBC's Barbara Plett in Beirut.
UK Foreign Secretary Jack Straw earlier warned Syria it risked being "treated as a pariah" if it ignored calls for withdrawal.
Mr Straw said members of the Security Council, of which the UK is one, had discussed deploying more UN peacekeepers to Lebanon in the event of a Syrian pullout.
Our correspondent says Syria is looking increasingly isolated as pressure mounts not only from the US and UK, but also from traditional allies like Russia, Saudi Arabia and France.
UN Security Council members have been considering measures against Syria since issuing a resolution in September 2004 calling for all foreign forces to leave Lebanon.
A 1989 agreement that ended the Lebanese civil war calls for a phased withdrawal of Syrian troops, beginning with redeployment to the eastern Bekaa Valley.