Damascus ordered the withdrawal on Monday, demanded by a UN Security Council resolution seven months ago, after coming under intense international pressure over the 14 February assassination of a Lebanese former PM Rafiq al-Hariri.
UN envoy Terje Roed-Larsen said Syrian Foreign Minister Faruq al-Shara had told him "all Syrian troops, military assets and the intelligence apparatus will have been withdrawn fully and completely ... by 30 April 2005".
Roed-Larsen was speaking at a joint news conference with al-Shara after talks with President Bashar al-Asad in Damascus.
"Syria has agreed that subject to the acceptance of the Lebanese authorities a UN verification team will be dispatched to verify [the full withdrawal]," said Roed-Larsen.
Lebanese opposition figures hailed the announcement, which fulfilled one of their key demands.
Al-Hariri's killing has plunged the
country into political turmoil
Speaking to Aljazeera by phone from Beirut, Lebanese opposition member of parliament Faris Buwayz said: "This step was expected and it has been heartily received in Lebanon.
"It will speed up the withdrawal and the measures required to transform Lebanon into an independent, sovereign and democratic state."
Syria first sent troops to Lebanon in 1976, early in its 1975-90 civil war, but in recent years had reduced numbers to about 14,000 from a peak of 40,000.
UN Resolution 1559, sponsored by the US and France, demanded the departure of all foreign forces, the disbanding of all Lebanese armed groups, and respect for Lebanon's political independence.
"Syria by its full withdrawal from Lebanon would have implemented its part of resolution 1559," al-Shara said.
The declared timetable means all Syrian forces will have left before Lebanon holds parliamentary elections. The polls were due to have taken place in May but might be pushed back because of political turmoil since al-Hariri's killing.
A Syrian journalist, speaking to Aljazeera, said that it would be hard to predict the effect of the withdrawal on Lebanon but that US plans would have an impact.
"There is a US agenda run by the neo-conservatives at the US administration which we do not know whether it will be ended or not," said Isam Dari, managing editor of the Syrian Tishrin newspaper, on Sunday.
He held out hope that relations between Syria and Lebanon would be friendly after the pullout.