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BEIRUT, Lebanon (CNN) -- A constitutional crisis in Lebanon is threatening plans for an international tribunal to try those allegedly involved in the assassination of the country's former Prime Minister.
Prime Minister Fouad Siniora indicated Sunday he would formally announce support for the tribunal at a cabinet meeting Monday.
But Lebanese President Emile Lahoud said Siniora's government lacks constitutional authority to make decisions because of Saturday's resignation of all five Shiite ministers from a 24-member cabinet.
Despite the resignations of the five Shiites -- two Hezbollah and three Amal ministers -- Siniora's cabinet still has 19 ministers, enough for the two-thirds majority required for the cabinet to act.
But because Lebanon's constitution requires all sects be represented in the cabinet, it is not yet clear whether their decisions would be legally valid.
Their withdrawal from the cabinet Saturday followed the collapse of negotiations that could have given Hezbollah more power in Lebanon's government.
Hezbollah had threatened to stage street demonstrations designed to topple Siniora's government unless a new "unity" administration was formed. That prompted several rounds of talks over the past week involving leaders of the major Lebanese political parties.
Hezbollah has been seeking to control at least a third of the cabinet, enough for veto power over the international tribunal.
Saad Hariri -- leader of the top Lebanese parliamentary bloc and son of the slain former Prime Minister Rafik Hariri -- said the ministers' resignations were part of a Syrian-Iranian plot to obstruct bringing his father's killers to justice.
"It's obvious that Iran and Syria has put huge amount of pressure on Hezbollah and Amal not to agree on this matter, and they tried to create a little bit of uncertainty in Lebanon," Hariri said about the Shiite parties.
"The Syrian regime is responsible for killing Rafik Hariri and all the others, so why protect those who kill Lebanese citizens, Lebanese people, leaders, religious leaders, journalists, members of parliament?" Hariri asked.
"I don't understand, and they (Syria) have to really wake up because the Lebanese people one day will not let them, will not excuse them for their actions."
White House Press Secretary Tony Snow said last week there was mounting evidence that Syria was working with Hezbollah to topple the Lebanese government in order to prevent establishment of the tribunal.
Lebanese Prime Minister Fouad Siniora may not have the authority to endorse a tribunal becuase of recent resignations of Shiites from his cabinet.