Damascus had argued for some months that it would not heed last September's UN resolution demanding the departure of all foreign troops, because Israel had yet to implement an array of other resolutions on the Middle East conflict.
However, the last Syrian soldier crossed the Lebanese border earlier on Tuesday, ending a 29-year deployment.
"The Syrian Arab Republic wishes to formally notify you that all Syrian military and security forces deployed in Lebanon went home today to Syria after carrying out several deployments in recent years," the letter from the Syrian Foreign Ministry said.
"The Syrian government assures the Security Council that the prompt implementation by its troops of Resolution 1559 is based on Syria's commitment to respecting the UN charter and its resolutions."
But the United Nations said it would await verification that Syria had pulled out of Lebanon, while acknowledging that other moves demanded by the Security Council had not yet taken place.
In a report released hours after Syria declared its withdrawal, Annan said the claim would be tested by a team of military experts who arrived in Damascus on Tuesday.
Syrian troops being welcomed
home on return from Lebanon
The team "will report to me and complete its work as soon as possible", Annan said, adding that no progress had been made on other key provisions of Resolution 1559.
And although US State Department spokesman Adam Ereli called the withdrawal an important first step towards Syria's compliance with the UN Security Council resolution, he also expressed doubts.
He said there were "lingering concerns" Syria had not withdrawn all its intelligence agents, adding that the Bush administration was looking forward to the report by the UN team sent to verify the withdrawal.
Civil war role
Damascus' Foreign Ministry pointed to what it called "Syria's positive contribution toward ending the (1975-1990) civil war in Lebanon and to preserving the unity of the country".
Syria also played a key role in building a new Lebanese army, it said.
The ministry urged the Security Council to work for the implementation of the UN's land-for-peace resolutions as the means to resolve the Arab-Israeli conflict.
"Syria has set the example," it said.
"It now urges the council to exert efforts with the same determination and seriousness to have the other resolutions implemented," it added, referring to Israel's occupation of Arab lands.