The White House on Wednesday sounded the alarm over what it called "mounting evidence" that Iran, Syria and the Hezbollah militia were "preparing plans to topple" Lebanon's government.
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.
Syria's embassy in Washington dismissed the accusations as "ludicrous" and "unfounded" and insisted that "Syria fully respects the sovereignty of Lebanon and does not interfere in its internal politics."
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 last year's murder of 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, Hezbollah and their Lebanese allies are preparing plans to topple Lebanon's democratically elected government, led by Prime Minister Siniora," he said.
"Any attempt to destabilize 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.
Snow's comments came after Hezbollah's leader, Hassan Nasrallah, accused Lebanon's government of promoting the occupation of Lebanon by the UN force policing a ceasefire 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.
"What is happening in Lebanon is a purely domestic political issue," Syria's embassy said in a statement that urged Washington "to stop instigating the Lebanese people against each other and against other countries."
The war of words 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.
The Druze leader notably complained of opposition to the court from Lebanon's pro-Syrian President Emil Lahoud.
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.