The United States has said there is "mounting evidence" that Syria, Iran and Hezbollah are planning to topple the Lebanese government.
The White House said Syria hoped to stop the formation of an international tribunal to try suspects in the killing of former Lebanese PM Rafik Hariri.
Spokesman Tony Snow said any attempt to destabilise the Lebanese government would violate UN resolutions.
A UN team has been investigating who was behind Mr Hariri's death in 2005.
The BBC News website's world affairs correspondent Paul Reynolds says the White House statement appears to result from the tense situation in Lebanon, where Hezbollah is demanding one third of cabinet seats, thereby giving it a veto over decisions.
Such a veto would enable it to block approval of the international tribunal to try suspects in Mr Hariri's assassination, our correspondent says.
The Hezbollah leader, Sheikh Hassan Nasrallah, has threatened street demonstrations in support of his demand.
The US is concerned that this instability could result in the fall of the government of Prime Minister Fouad Siniora.
The statement also casts doubt on any willingness by the Bush administration to consider Syria and Iran as potential partners over the future of Iraq, an idea that the Baker commission on Iraq is expected to suggest, our correspondent adds.
Wednesday, November 01, 2006
Lebanon Plot Warning
Via the BBC: