Lebanon was steeped in tension on Saturday after Hezbollah seized control of west Beirut in what the ruling Western-backed coalition branded an armed coup orchestrated with the help of Iran and Syria.
At least 18 people were killed in the fighting that pitted Sunni supporters of the ruling bloc against Shiite members of the opposition and brought the country dangerously close to a new civil war.
Washington blasted the power grab by Hezbollah and also pointed the finger at Iran and Syria for the unrest, saying they must be held to account.
"The United States is consulting with other governments in the region and with the UN Security Council about measures that must be taken to hold those responsible for the violence in Beirut accountable," the White House said.
A senior US official declined to list any specific steps, and emphasised that there would not be a "one-size-fits-all" approach to Tehran, Damascus, and the Shiite militant group that Washington brands a terrorist organisation.
An uneasy calm settled over west Beirut on Saturday as people ventured out in the affected areas and cleaning crews swept away the debris.
Few armed elements could be seen on the streets and the army was out in force manning roadblocks. However, the main road leading to the airport remained blocked for a fourth straight day and there were no scheduled incoming or outgoing flights.
"The presence of armed elements has significantly decreased and there is no danger any more for civilians," an army spokesman told AFP.
Foreigners, meanwhile, continued leaving the country by road to Syria, with Turkey and Kuwait evacuating their citizens on Saturday.
The unrest led to urgent international appeals for calm as Arab foreign ministers prepared to hold an emergency meeting on the crisis on Sunday amid fears among Sunni governments of an increase in the influence in Lebanon of Shiite Iran.
The fighting was sparked by the government's decision to probe a communication network set up by Hezbollah and to sack the head of airport security over his alleged links with the militant group.
The violence that ensued between Sunni and Shiite militants marked a turning point in the 18-month political crisis that has virtually paralysed the government of Prime Minister Fuad Siniora and left the country with no president.
"Will Siniora go back on his decisions or will he resign and resolve the country's crisis?" asked the opposition newspaper Ad-Diyar on Saturday.
"Political impasse and insecurity – Hezbollah's pyrrhic victory," said the pro-government L'Orient Le Jour.
Analysts said there were fears the violence could spread beyond Lebanon's borders.
"Hezbollah had said before it would never point its arms against its own country, but it has crossed this line," said Nadim Shehadi, Lebanon expert at the Chatham House think-tank in London.
"Hezbollah has crossed a huge red line, and it means Iran has crossed this line. The political consequences are immense – it could escalate regionally."
Both sides appeared unwilling to give any further ground, with the opposition insisting that the roadblocks that have paralysed west Beirut and the airport road would remain until the government meets its demands.
"We are not carrying out a coup – all of this is related to the government's decisions," an opposition official told AFP. "We are offering partnership... and they want to monopolise power and limit our share."
But Youth and Sports Minister Ahmed Fatfat ruled out any chance of the government going back on its decision to probe the communications network that Hezbollah insists is essential to its defences against Israel.
"It would be easier for the government to resign than to revoke its decision," Fatfat told AFP.
Lebanon's long-running political standoff, which first erupted in November 2006 when six pro-Syrian ministers quit the cabinet, has left it without a president since November, when Damascus protege Emile Lahoud stepped down at the end of his term of office.
Syrian President Bashar al-Assad said the unrest was a purely "internal affair" but called for dialogue.
Lebanon's crippling political divide is widely seen as an extension of the confrontation pitting the United States and its Arab allies against Syria and Iran.