The planned court is a bone of contention between Lebanon's Western-backed government and the opposition, spearheaded by the Hezbollah guerrilla group, which is holding round-the-clock protests to try to oust the anti-Syrian prime minister.

Hundreds of thousands of opposition protesters are expected to stage a mass rally in downtown Beirut on Sunday as part of their campaign to oust Prime Minister Fouad Siniora.

A U.S. State Department official on Saturday accused Hezbollah allies Syria and Iran of trying to destabilise Lebanon and said the situation was of "very significant concern".

Speaking in Kuwait, U.S. Deputy Assistant Secretary of State James Jeffrey said Washington was trying to mobilise moderates in the Middle East to support the Lebanese administration.

Six pro-Syrian ministers, including five Shi'ite Muslims, quit the cabinet last month to press demands for a government of national unity.

The remaining ministers subsequently approved proposals for the tribunal, but Lahoud said in a statement on Saturday that the depleted cabinet was unconstitutional and therefore did not have the authority to pass the court project. "The president ... calls on the cabinet to reconsider (the plan) when there is a legitimate and constitutional government," a statement from his office said.

The move had been widely expected and leaves the court project hanging in the balance.

Hariri was killed by a truck bomb in February 2005 -- an assassination many government supporters blame on neighbouring Syria, a charge Damascus denies.

Plans for the international tribunal were drafted by the United Nations and Beirut after Siniora said Lebanon's judiciary could not handle the case alone.

Siniora's allies say the Shi'ite Hezbollah group is trying to sink the government in order to scupper the tribunal.

The government is now expected to refer the court plans to parliament, where it has a Sunni-led majority, in a bid to bypass the president. However parliamentary speaker Nabih Berri, who is allied to Hezbollah, has indicated he will not authorise a debate on this.

Commentators have warned that the increasingly acrimonious stand-off could degenerate into violence in a country that has suffered two civil wars in 50 years.