BEIRUT (BBC) – A sixth minister has resigned from the Lebanese government, which was plunged into a political crisis when all five Shia cabinet members quit. Environment Minister Yacoub Sarraf, a Christian, is an ally of pro-Syrian President Emile Lahoud like his Shia colleagues from Hezbollah and Amal.

They resigned after calls for a greater role in government were rejected. But a cabinet meeting to consider a UN plan to try killers of former PM Rafik Hariri went ahead despite the ructions. "I don’t see myself belonging to any constitutional authority in which an entire sect is absent," Mr. Sarraf said in his letter of resignation, according to Lebanon’s National News Agency.

Mr. Lahoud said on Sunday that as a result of the resignations, the government had lost its legitimacy – but constitutional experts have disputed his interpretation of the situation. The cabinet, normally made up Christian and Muslim ministers in equal numbers, has retained the two-thirds of its members necessary to make up a quorum.

Prime Minister Fouad Siniora has not accepted the resignations, but the ministers insist they will stand by their decisions.

Correspondents say the fact that Mr. Sarraf is a Christian, strengthens the Shias’ bid for a larger presence in cabinet, and reduces the sectarian nature of the dispute.

Hezbollah, whose fighters gave Israel a bloody nose during the 34-day conflict in the summer, is seeking a one-third-plus-one share of cabinet portfolios for itself and its allies, giving them an effective veto power on government decisions.

Another two ministers would need to resign for the current government to fall.

Leaders of the anti-Syrian parliamentary majority said the latest moves revealed a "hidden plot" by Syria to stop the establishment of the Hariri tribunal and foil UN resolution 1701, which halted the conflict between Hezbollah and Israel in August.

Hezbollah denies trying to obstruct the Hariri tribunal, but says it wants to discuss its details.