Lebanon's Iranian-backed Hezbollah group took control of the Muslim half of Beirut on Friday in what the U.S.-backed governing coalition described as "an armed and bloody coup", Reuters reported.
The United States pointed to Hezbollah's links with Iran and Syria, and said it was talking with other powers about taking measures against "those responsible for the violence".
At least 18 people have been killed and 38 wounded in three days of battles between pro-government gunmen and fighters loyal to Hezbollah, a Shi'ite movement with a powerful guerrilla army.
The violence follows 17 months of political deadlock between the Hezbollah-led opposition, which demands more say in government, and the ruling coalition. It has paralysed the country and left it without a president.
Lebanon's worst internal strife since the 1975-90 civil war erupted this week after the government decided to dismantle Hezbollah's military communications network. The group said the government had declared war.
In scenes reminiscent of the civil war, men with rifles roamed the streets amid smashed cars and smouldering buildings.
Fighting died down as outgunned government supporters handed over their weapons and offices to the army, which has tried to remain neutral in the conflict.
SYRIA AND IRAN
The anti-Syria ruling coalition said the "armed and bloody coup" was aimed at increasing Iran's influence and restoring that of Syria, forced to withdraw troops from Lebanon in 2005.
A U.S. State Department spokesman said Secretary of State Condoleezza Rice had phoned Saudi Foreign Minister Prince Saud al-Faisal, French Foreign Minister Bernard Kouchner and U.N. Secretary General Ban Ki-moon to discuss "what the international system can do to support this Lebanese government ... in the face of illegal acts, by the armed gangs".
"It is also becoming more apparent now that the linkages that we know exist, and are ongoing, between Hezbollah and Syria and Iran, are starting to manifest themselves in the current crisis," he said.
"We are seeing now some evidence of those groups that are linked to Syria, that are in Lebanon right now, are taking a much more active role in fanning the flames of the violence."
A White House spokesman said: "The United States is consulting with other governments in the region and with the U.N. Security Council about measures that must be taken to hold those responsible for the violence in Beirut accountable."
The White House restated its support for the government and urged Iran and Syria to end support for Hezbollah.
"This support is a reflection of our unshakable commitment to the Lebanese people and their hope for democratic change," U.S. Secretary of State Rice said.
A senior opposition source told Reuters that Hezbollah and its allies would maintain their roadblocks, including barricades on routes to the airport, until a full resolution of the crisis.
"All issues are linked. Beirut will remain shut until there is a political solution," the source said.
An influential pro-government leader called for dialogue.
Walid Jumblatt, leader of the Druze minority, said Hezbollah "regardless of its military strength, cannot annul the other".
KILLED TRYING TO FLEE
The dead included a woman and her 30-year-old son, killed while trying to flee Ras al-Nabae -- a mixed Sunni-Shi'ite Beirut district and scene of some of the heaviest clashes.
"They were trying to flee to the mountains. Instead ... they reached the hospital, dead," said a relative, who declined to give her name out of fear for her own safety.
"It was terrifying during the night. We couldn't even move about in the house," said another woman, a Ras al-Nabae resident who fled the area at first light with her children. "We spent the night in the corridor."
Hezbollah seized the offices of pro-government factions, including the Future group of Sunni politician Saad al-Hariri, in the predominantly Muslim western half of the city.
Backed by gunmen from the Shi'ite Amal group, Hezbollah handed over the offices to the army. Hariri supporters gave up their offices to the army elsewhere in the country.
Hezbollah also moved into Hariri-owned media outlets, and Hariri's television and radio stations went off the air. Opposition gunmen of the Syrian Socialist National Party set ablaze a building housing studios of Hariri's TV station.
"It certainly leaves the government weaker and the Future movement weaker," said Paul Salem, director of the Carnegie Middle East Center in Beirut.
But Hezbollah does not want to be seen as an "occupier of Beirut", he said, and handing control to the army appeared its most likely exit.
The European Union, Germany and France urged a peaceful resolution. Syria said the issue was an internal Lebanese affair while Iran blamed the United States and Israel for the violence.
Hezbollah was the only Lebanese faction allowed to keep its weapons after the civil war to fight Israeli forces occupying the south. Israel withdrew in 2000 and the fate of Hezbollah's weapons is at the heart of the political crisis.