Lebanese Prime Minister Najib Mikati announced the resignation of his government on Friday, citing divisions on several domestic issues, and called for the formation of a national unity government.
"I announce the resignation of the government, hoping that this will open the way for the major political blocs to take responsibility and come together to bring Lebanon out of the unknown," Mikati said.
He called for the "formation of a national salvation government in which all Lebanese political forces are represented in order to save the nation and deal with regional developments with a collective spirit of responsibility."
His resignation comes as Lebanon buckles under the pressure of the conflict engulfing neighbouring Syria, which has exacerbated existing tensions in Lebanon's multi-confessional population.
The violence between opponents and supporters of President Bashar al-Assad has already spilled over into Lebanon, and Damascus has warned Beirut against allowing fighters and weapons to flow over the border.
In Washington, State Department spokeswoman Victoria Nuland, speaking before Mikati resigned, told journalists that the United States was watching the situation in Lebanon "very, very closely."
"Our basic view of this is that we believe the Lebanese people deserve a government that reflects their aspirations and that will strengthen Lebanon's stability, its sovereignty and its independence," she said.
"And we have grave concerns about the role the Hezbollah plays," she said of the Shiite militant movement dominant in the Mikati government.
Mikati's decision, which automatically brings down the government, came after disagreements on two domestic issues, the formation of an elections supervisory commission and the extension of a security chief's mandate.
The government has held off on agreeing on the membership of the commission over fears it would ensure that elections scheduled for June are held on the basis of a decades-old electoral law.
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Mikati, along with the leader of Lebanon's Druze community, Walid Jumblatt, is said to favour the existing law.
It gives his Sunni community and the Druze disproportionate strength in parliament, but is vehemently opposed by Lebanon's Christians, who say it fails to give them representative weight.
Attempts earlier this year to approve an alternative election law failed, and both Mikati and President Michel Sleiman have called for preparations to move forward so the vote can be held on time.
The 57-year-old's resignation also came against the backdrop of his contentious bid to extend the term of the Sunni head of Lebanon's Internal Security Forces, General Ashraf Rifi, which was opposed by a majority of the government.
Miqati said he was willing to resign last year, after a car bombing that killed the police intelligence chief, but Sleiman rejected it and he stayed in office.
He became prime minister in 2011, after five months of negotiations, positioning himself as a political moderate able to deal with all political parties.
He headed a government dominated by the so-called March 8 coalition, made up of Hezbollah and its allies, and drew fire from Sunnis who accused him of betraying his community and siding with the Syrian- and Iranian-backed group.
The resignation throws Lebanon into new uncertainty, and comes as the violence in Syria increasingly affects the country.
Syrian warplanes bombed targets inside Lebanon for the first time on Monday, after Damascus warned that it would not tolerate the flow of weapons and fighters across its borders.
There has also been violence in the northern city of Tripoli, with six people killed this week in battles pitting pro- and anti-Damascus neighbourhoods against each other.