|Hezbollah says the UN tribunal over the murder of former premier Rafiq al-Hariri has been politicised [EPA]
Lebanon's unity government has collapsed after the Hezbollah movement and its political allies resigned from the cabinet over arguments stemming from a UN investigation into the assassination of Rafiq al-Hariri, the former Lebanese prime minister, in 2005.
There has been growing political tension in Lebanon amid signs that Hezbollah members could be indicted by the UN-backed Special Tribunal for Lebanon (STL).
Ten ministers tendered their resignations on Wednesday after reports that al-Hariri's son Saad, the prime minister, had refused their call to convene a cabinet meeting to discuss controversial issues including the investigation.
An eleventh member, Adnan Sayyed Hussein, later stood down from the 30-member cabinet, automatically bringing down al-Hariri's government.
The request to convene a cabinet meeting came on Tuesday after Syria and Saudi Arabia, who have for months been attempting to act as mediators in Lebanon's political crisis, announced their efforts had failed.
The standoff between al-Hariri's camp and Hezbollah over the UN tribunal has paralysed the government for months and sparked concerns of sectarian violence similar to the one that brought the country close to civil war in May 2008.
Visit cut short
The resignations, which were announced by Jubran Bassil, the energy minister, came as al-Hariri was in Washington meeting Barack Obama, the US president.
Bassil called on Michel Suleiman, the Lebanese president, to form a new government.
Immediately after his government was brought down, al-Hariri cut short his visit to the US to fly home.
He was driven to Dulles international airport after talks with Obama at the White House and will meet Suleiman on his return. He will be stopping in Paris on his way back to meet Nicolas Sarkozy, the French president.
The White House later accused Hezbollah of acting out of "fear" and commended Hariri "for his steadfast leadership and efforts to reach peace, stability, and consensus in Lebanon under difficult circumstances".
Hezbollah, which has denied any role in the assassination, has denounced the tribunal into the 2005 killing as an "Israeli project'' and urged al-Hariri to reject any findings by the court, which has not yet announced its decision on the indictments.
Al Jazeera's Rula Amin, in Beirut, said that the opposition ministers said that they resigned because the prime minister will not engage them and respond to efforts to reach an understanding on a number of issues.
"What the ministers are saying is that they resigned to protest Saad Hariri's stand not to find a settlement on how to deal with the ramifications of the tribunal.
"Hezbollah are saying that this tribunal has been politicised and used by the US and Israel to discredit Hezbollah, that they have nothing to do with the death of Rafiq Hariri and they want Saad Hariri to come out and delegitimise its findings and end Lebanon's co-operation with the tribunal."
Hariri has refused to break co-operation with the tribunal.
"Saad Hariri was on the brink of making a major concession as concerns the tribunal but occult forces prevented him from doing so," Walid Jumblatt, the Druze leader, told the AFP news agency without elaborating.
Saad Hariri's associates said that he would not succumb to pressure and will call for a cabinet meeting when he finds it appropriate. The prime minister himself is still to comment on the fall of his government.
Reacting to the Hezbollah withdrawal, Hillary Clinton, the US secretary of state, said the tribunal must continue with its work so that justice can be served.
"We view what happened today as a transparent effort by those forces inside Lebanon, as well as interests outside Lebanon, to subvert justice and undermine Lebanon's stability and progress," Clinton told a news conference in Doha, Qatar, where she was attending a meeting of regional leaders.
"This is a matter that should be allowed to proceed as previously agreed to. This is not only about the tragic assassination of former Prime Minister Hariri, but many other people died and were injured as well," Clinton said.
In a statement, a spokesperson for Ban Ki-moon, the UN secretary general, said: "The secretary general is monitoring closely developments in Lebanon, where the situation is fast evolving. He emphasises the importance that calm be preserved.
"The secretary-general further calls for continuing dialogue among all parties and respect for the constitution and the laws of Lebanon.
"He reiterates his full support for the independent work of the Special Tribunal for Lebanon."