Syrian forces have been in Lebanon since 1976

Speaking after talks in Damascus, William Burns also called on Syria to sever ties with Islamist militants.

Syria has 15,000 troops in Lebanon and allegedly backs the Palestinian group Hamas and Lebanon's Hezbollah.

The US imposed trade sanctions on Syria this year over these alleged ties and weapons programmes.

If Syria takes action on these concerns, our relationship can take a very different course with positive results William Burns UN Resolution 1559

Mr Burns also raised concern over Syria's perceived failure to prevent anti-US militants entering Iraq via its border.

The BBC's Kim Ghattas reports from Damascus that Mr Burns is the first senior US envoy to visit Syria in a year and the trip comes when relations are at an all-time low.

UN demand

Speaking after talks with President Bashar al-Assad, the envoy held out the possibility that relations could be improved if Damascus met Washington's wishes.

Burns issued his demand after meeting the Syrian leader

"If Syria takes action on these concerns, our relationship can take a very different course with positive results," he said.

But he stressed that America expected Syria to withdraw from Lebanon in line with United Nations Security Council Resolution 1559, which was sponsored by the US and French and passed last week.

"Syria must end its interference in Lebanese internal affairs, withdraw its forces from Lebanon and allow the Lebanese armed forces and government to establish their authority throughout Lebanon ," Mr Burns said.

"The time has come for concrete steps... We stress that what is essential now is genuine progress not rhetoric."