The White House charged Wednesday that "mounting evidence" shows Syria and Iran are joining with Hizbullah in plans to topple the Lebanese government.
In an unusually strong statement, press secretary Tony Snow said any attempt to manufacture demonstrations or use violence or physical threats against Lebanese leaders would be a clear violation of Lebanon's sovereignty and of three UN Security Council resolutions.
Snow said there were indications that Syria wants to prevent the government of Prime Minister Fuad Siniora from approving the statute for an international tribunal that would try those accused of involvement in former Prime Minister Rafik Hariri's assassination in 2005.
"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," Snow said. He added that the United States is committed to the quick establishment of a tribunal.
A UN investigation into Hariri's killing has implicated top Syrian and Lebanese security officials, a charge Syria denies. The assassination sparked massive street protests and prompted Syria to withdraw its forces from Lebanon under international pressure.
Snow said Washington was "increasingly concerned by mounting evidence" that Syria, Iran and Hizbullah "are preparing plans to topple Lebanon's democratically elected government."
The statement provided no specific details about the stated evidence.
When asked about the statement later by reporters, Snow said: "We're making it clear to everybody in the region that we think it ought to be hands off the Siniora government and let them go about and do their business."
The White House announcement came as Hizbullah threatened street protests to force early elections in Lebanon.
Hizbullah is demanding creation of a "National unity" cabinet that would give the Islamic group and their allies veto power over key decisions.
The United States has been concerned about Syrian and Iranian actions in Lebanon for some time.
At the United Nations on Monday, US Ambassador John Bolton accused the two countries of trying to destabilize Lebanon's democratically elected government by violating a UN Arms embargo.
Bolton stressed that Syria's obligations to respect a UN arms embargo authorized in August "are particularly important, as it is the one country other than Israel that borders Lebanon."