America delivered a blunt message to Syria and its allies yesterday that any attempt to “topple” the Lebanese government would result in failure.

Tony Snow, the White House spokesman, said there was mounting evidence that the Damascus regime and its Iranian allies in Tehran were backing Hezbollah’s attempt to bring down the Lebanese government in order to derail an international tribunal into the assassination last year of Rafik Hariri, the former Lebanese prime minister.

“Support for a sovereign, democratic, and prosperous Lebanon is a key element of U.S. policy in the Middle East,” Mr. Snow said.

He said America was “increasingly concerned” that plans were being prepared to bring down the Beirut government led by Prime Minister Siniora.

“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,” he said.

Hezbollah, the Shiite movement whose capture of Israeli soldiers sparked war in the summer, has threatened to use street protests to force the creation of a “national unity” government that would give it sufficient power to veto key legislation. The group’s leader, Hassan Nasrallah, has also accused Lebanon’s government of promoting the occupation of the country by the United Nations force policing a ceasefire with Israel.

Israel has signaled its disquiet over developments in Lebanon by sending warplanes on sorties over Beirut.

The White House warning came as Washington was digesting reports that Tony Blair’s senior foreign policy adviser visited Damascus for the highest-level meeting between Britain and Syria since the invasion of Iraq.

Sir Nigel Sheinwald met President Assad and Walid al-Moallem, his foreign minister, on Monday, fueling speculation that the prime minister plans to visit Syria himself.

But a senior Lebanese politician said yesterday that any talks between Mr. Blair and Mr. Assad would only serve to reverse the progress of democracy in the Middle East and do nothing to stabilize the situation in Lebanon or Iraq.

Mr. Sheinwald’s trip is being seen as a means of putting pressure on Syria to play a positive role in bringing stability to Iraq.

Syria is considered a key regional player due to its role in allowing militants to cross its border to join the Iraqi insurgency, its support for Hezbollah, and its influence over the Hamas leadership, which is partly based in Damascus.