But spokesman Tony Snow refused to provide details or even describe the information underpinning the accusation, saying it was classified and that keeping the charge vague "serves a diplomatic purpose and an important one."
"We're making it clear to everybody in the region that we think that there ought to be hands off the Siniora government; let them go about and do their business," he said, referring to Lebanese Prime Minister Fuad Siniora.
Earlier, Snow said in a statement that Damascus apparently hoped to derail efforts to set up an international tribunal to try those accused of taking part in the 2005 murder of former Lebanese prime minister Rafiq Hariri.
"Any such effort to sideline the tribunal will fail, however, for the international community can proceed with establishing it no matter what happens internally in Lebanon," said the spokesman.
"The United States is committed to working with its international partners and the legitimate government of Lebanon to ensure that the tribunal is quickly established and that all those responsible for the assassinations of Rafiq Hariri and other Lebanese patriots since 2005 are brought to justice," he said.
"Support for a sovereign, democratic and prosperous Lebanon is a key element of US policy in the Middle East," Snow said in a statement.
"We are therefore increasingly concerned by mounting evidence that the Syrian and Iranian governments, Hizbullah and their Lebanese allies are preparing plans to topple Lebanon's democratically elected government, led by Prime Minister Siniora," he said.
"Any attempt to destabilise Lebanon's democratically elected government through such tactics as manufactured demonstrations and violence, or by physically threatening its leaders, would, at the very least, be a clear violation of Lebanon's sovereignty" and UN resolutions, said Snow.
The statement came two days after Lebanon's Druze leader Walid Jumblatt sought US backing for the international court to try suspects in Hariri's slaying.
Jumblatt said he discussed the proposed tribunal with US Secretary of State Condoleezza Rice during an unannounced meeting here.
The Druze leader notably complained of opposition to the court from Lebanon's pro-Syrian President Emil Lahoud.
"If someone opposes this international court, that means that he is covering up the crime," Jumblatt, a leading anti-Syrian figure in Lebanon's parliament, told reporters after meeting Rice at the US State Department.
A draft text on the project was sent to Lebanese authorities by the UN on October 21. The tribunal has yet to be approved by the UN Security Council or by Lebanon's cabinet and parliament.
The idea for the international tribunal, which would meet outside Lebanon for security reasons, was floated in March by UN Secretary General Kofi Annan.
Hariri was assassinated on February 14, 2005 in a massive bomb blast on the Beirut seafront that killed 22 others. The popular five-time prime minister had opposed the three-year extension of Lahoud's mandate, pushed through by Syria in 2004.
Snow's comments came after Hizbullah's leader, Hassan Nasrullah, accused Lebanon's government of promoting the occupation of Lebanon by the UN force policing a cease-fire between his Shiite group and Israel.
The militia is calling for the formation of a government of national unity, and warning that it may call demonstrators into the streets if it does not get its way.

Copyright AFP (Agence France-Presse), 2006