President Barack Obama's Middle East envoy, George Mitchell, has arrived in Syria for talks with officials including President Bashar al-Assad.
He is the highest-ranking US diplomat to visit Damascus since Barack Obama's administration took office.
Correspondents say the US is testing Syria's support for America's stated new drive for peace in the region.
But Mr Mitchell has assured Lebanon that securing Syrian co-operation will not come at Beirut's expense.
He arrived in Syria from Beirut, and has already visited Israel, the West Bank, Egypt and Jordan.
Mr Mitchell is set to meet President Assad on Saturday morning, after which he will deliver a statement.
The BBC's Lina Sinjab in Damascus says Syrians are looking forward to the meeting.
Improved relations could mean a lifting of sanctions and many new business deals - something many ordinary Syrians are hoping for.
However, our correspondent says, the government and people here want to see pressure from the superpower on the Israelis to deliver long-awaited promises of peace.
The visit comes just a week after President Obama's ground-breaking speech in Cairo, in which he called for a "new beginning" between Muslims and the US, and vowed to pursue aggressively an end to the Israeli-Palestinian conflict.
He did not mention Syria in the speech, but Damascus, where the United States still does not have an ambassador, remains a key regional player, says the BBC's diplomatic correspondent Jonathan Marcus.
It has an influence over events in Lebanon; it shares a crucial border with Iraq; it has a significant relationship with the Islamist Palestinian movement Hamas which controls the Gaza Strip; and it is a close ally of Iran, he says.
But above all it wants Syria to push Hamas along the road to Palestinian unity - and ultimately Washington would like to woo Damascus away from Tehran.
Syria also has issues: It wants to get back the Golan Heights, territory occupied by Israel in the 1967 war. It also wants to see an end to US economic sanctions.
While in Beirut, Mr Mitchell stressed that Mid-East peace could not be achieved "at Lebanon's expense", reported the news agency AP.
Factions in Lebanon are concerned that improving relations could see Syria reassert its influence in Lebanon.
Under international pressure, Syria withdrew soldiers from Lebanon following the 2005 assassination of former Lebanese Prime Minister Rafik Hariri.
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