Lebanon's national unity government has collapsed after Hezbollah ministers and their allies resigned over a United Nations-backed tribunal investigating the assassination of former prime minister Rafik Hariri.

The walkout ushers in the country's worst political crisis since 2008 in one of the most volatile corners of the Middle East.

The tribunal is widely expected to name members of Hezbollah in upcoming indictments, which many fear could re-ignite sectarian tensions that have plagued the tiny country for decades.

Lebanese opposition minister Jibran Bassil, centre, announces he is quitting the cabinet as two other resigning ministers, Mohammad Khalifeh, right, and Hussein Haj Hassan, listen at a news conference in Rabieh, near Beirut, on Wednesday. (Sharif Karim/Reuters)"This cabinet has become a burden on the Lebanese, unable to do its work," Energy Minister Jibran Bassil said at a news conference announcing the resignations, flanked by the other ministers who are stepping down. "We are giving a chance for another government to take over."

Hezbollah, which is backed by Iran and Syria, has denounced the tribunal as an "Israeli project" and urged Western-backed Prime Minister Saad Hariri, the son of the slain politician, to reject any findings by the court even before it announces any indictments.

But the prime minister has refused to break co-operation with the tribunal.

Saad Hariri was in Washington on Wednesday to meet with President Barack Obama, but he cut short his visit to return to Lebanon.

Diplomatic failure

The walkout followed the failure of a diplomatic push by Syria and Saudi Arabia to ease political tensions in Lebanon. There had been few details about the direction of the Syrian-Saudi initiative, but the talks were lauded as a potential Arab breakthrough, rather than a solution offered by Western powers.

Bassil said the ministers decided to resign after Hariri "succumbed to foreign and American pressures" and turned his back on the Syrian-Saudi efforts.

Calls to the tribunal seeking comment Wednesday were not immediately returned.

Hariri formed the current national unity government in November 2009, but it has struggled to function amid deep divisions. The crisis over the tribunal has paralyzed the government in recent months.

Rising tensions

Violence has been a major concern in Lebanon, where Shias, Sunnis and Christians each make up about a third of the country's four million people. In 2008, sectarian clashes killed 81 people and nearly plunged Lebanon into another civil war.

Rafik Hariri's assassination in a suicide bombing that killed 22 other people both stunned and polarized Lebanese. He was a Sunni who was a hero to his own community and backed by many Christians who sympathized with his efforts in the last few months of his life to reduce Syrian influence in the country. A string of assassinations of anti-Syrian politicians and public figures followed, which UN investigators have said may have been connected to the Hariri killing.

The Netherlands-based tribunal has not said who it will indict, but Hezbollah leader Sheik Hassan Nasrallah has said he has information that members of his group will be named.