The United States has held up Lebanon as an example of emerging democracy in the Middle East.

"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," White House spokesman Tony Snow said in a statement.

He later told reporters, "We're making it clear to everybody in the region that we think that there ought to be hands off the (Prime Minister Fouad) Siniora government; let them go about and do their business."

Syria's embassy in Washington denounced the comments as "ludicrous" and "unfounded."

"What is happening in Lebanon is a purely domestic political issue," the Syrian Embassy said in a statement. "Syria fully respects the sovereignty of Lebanon and does not interfere in its internal politics."

The statement also urged Washington to stop meddling in Lebanese politics and "to stop instigating the Lebanese people against each other and against other countries."

US officials declined to give evidence for the accusations, saying the information was classified.

Hezbollah and its allies have been demanding a new government since a 34-day war between Israel and the Shia guerrillas ended in August. Hezbollah, backed by Syria and Iran, has been a fierce critic of Western-backed Siniora, whom it sees as keen to disarm its guerrillas.