United Nations chief Ban Ki-moon said he was "deeply concerned at the fragility of the situation in Lebanon" in a statement, as the United States warned its citizens in Lebanon of possible unrest.

Ban urged "all parties to maintain calm as well as to further intensify efforts to reach a compromise as soon as possible" and "to act within the constitutional framework as well as in a peaceful and democratic manner."

The US authorities "urge all Lebanese political groups to do their part to maintain calm and promote security for Lebanon’s citizens," the State Department said in a statement.

Lebanon was left facing a dangerous power vacuum and further political chaos as its President Emile Lahoud stepped down Friday night when his mandate expired, after rival politicians failed to elect a successor.

The European Union earlier voiced "regret" at the failure and urged political factions to strive for a peaceful solution.

A statement by Portugal, current holder of the EU’s rotating presidency, urged "all concerned to respect the Lebanese constitution and to abstain from all actions that could upset public order and the security of the citizens."

With tanks and troops on the streets of Beirut to maintain security, lawmakers from the Western-backed majority and the Hezbollah-led opposition had been scheduled to convene on Friday in a final bid to pick a new president.

But the session was postponed for the fifth time in two months, threatening to plunge Lebanon into further chaos.

The current crisis its worst since the country’s prolonged 1975-1990 civil war.

Lahoud stepped down as scheduled despite the inability of the pro- and anti-Syrian camps to replace him. His office said he had handed over security to the army.

"There is a strong possibility for demonstrations and unrest during this period," the State Department warned. "The US embassy urges US citizens who live, work, or are traveling in Lebanon to exercise responsible security practices."

The United States pledged support for Lebanon in the crisis, calling for continued talks to elect a new president.

"The United States and its allies will not waver in our support for the people of Lebanon as they defend their freedom against all attempts at foreign interference and intimidation," it said. -AFP