The United States voiced concern over Syria's military build-up at its northern border and said the recent massive bomb attack in Damascus must not be used as a pretext to get its forces back into Lebanon.
State Department spokesman Robert Wood said the United States and others had made very clear to Syria that any intervention by Damascus into Lebanon was unacceptable.
"The recent terrorist attacks that took place in Tripoli (Lebanon) and Damascus should not serve as a pretext for, you know, further Syrian military engagement or, should not be used to interfere in Lebanese internal affairs," Wood told reporters.
Syria tightly controlled politics and security in Lebanon until the 2005 assassination of former Prime Minister Rafik al-Hariri. The murder triggered international pressure that ultimately forced Syria to end its 29-year military presence in Lebanon and pull its forces back.
Damascus has warned of growing Islamist militancy in north Lebanon and the Syrian authorities have said a vehicle used in a suicide attack in Damascus last month crossed into the country from a neighboring Arab state. Along with Lebanon, Syria's Arab neighbors are Iraq and Jordan.
At the end of last month, Syria sent hundreds of troops to its northern border with Lebanon, in a move the authorities said was aimed to combat smuggling.
Syria's opponents in Lebanon have speculated that Damascus could use insecurity in the north as a pretext for intervention.
"Obviously we're concerned about this type of activity along the border and that it not lead to any further interference on the part of Syria into Lebanon's internal affairs," Wood said.
"The Syrian government is well aware of our views with regard to any kind of military activity along the border," he added.
US Secretary of State Condoleezza Rice met Syrian Foreign Minister Walid al-Moualem late last month on the sidelines of the UN General Assembly and pressed him on the Lebanon border build-up and other issues.
Two days later, the State Department's key Middle East policy diplomat met Moualem again to discuss that and other issues.
While critical of Syria over what it sees as interference in Lebanon, a senior US official told Reuters last week that Washington was reassessing its isolation strategy towards Damascus and how best to get Syria to change its behavior.
So far, US strategy has been to impose a raft of sanctions against Syria, which is also on the US list of state sponsors of terrorism.