The White House said Wednesday that Syria, Iran and Hezbollah are trying to destabilize and possibly topple the elected government of Lebanon.

In a statement e-mailed to reporters, White House press secretary Tony Snow said Washington was "increasingly concerned by mounting evidence that the Syrian and Iranian governments, Hezbollah and their Lebanese allies are preparing plans" to force the government of Prime Minister Fouad Siniora from power.

"We're making it clear to everybody in the region," Snow later told reporters, "that there ought to be [a policy of] hands off the Siniora government; let them go about and do their business."

Snow declined to give evidence for the U.S. concern, saying it was based on classified information.

Hezbollah, one of the largest political parties in the Lebanese parliament, has been demanding the appointment of a new cabinet, and threatening to stage street protests that would force fresh elections.

The move is seen as a way to capitalize on the organization's popularity in the wake of fighting with Israel last summer, from which Hezbollah says it emerged victorious.

Hezbollah is largely supported by Lebanese Shia Muslims and has close ties to Damascus and to Tehran.

The Syrian embassy in Washington dismissed the concerns expressed by the White House as "ridiculous" and American interference in "a purely domestic political issue."

Hezbollah and its allies in Parliament are demanding at least a third of the seats in a new cabinet. That would give them veto power over legislation and government policy, including a decision on approving an international tribunal to investigate the assassination in 2005 of former prime minister Rafik Hariri.

A UN-led inquiry last year implicated Syria in the murder, but Damascus has denied involvement.