The White House said it was "very troubled" by Hezbollah's actions in Beirut and urged Iran and Syria to halt support for the militant group after its fighters in the Lebanese capital routed forces loyal to the government.
The Bush administration also said it was conferring with the UN Security Council and other regional governments on possible action against Hezbollah for the violence that has racked Beirut for days.
"After months of paralyzing the democratically elected Lebanese government, Hezbollah has now turned its arms against the Lebanese people and challenged Lebanon's security forces for control of the streets," White House spokesman Gordon Johndroe said in a statement.
"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," he said.
Earlier on Friday, Johndroe told reporters, "We are very troubled by the recent actions of Hezbollah."
US Secretary of State Condoleezza Rice telephoned leaders in the region over events in Lebanon and also spoke to the country's pro-Western prime minister, Fouad Siniora.
"The secretary said the prime minister was very strong in their conversation. He remains resolute. He is in his office, he is working," said a senior State Department official.
Rice had harsh words for Hezbollah and said US support for Siniora's government was "unwavering".
"Backed by Syria and Iran, Hezbollah and its allies are killing and injuring fellow citizens, undermining the legitimate authority of the Lebanese government and the institutions of the Lebanese state," Rice said in a statement.
"We will stand by the Lebanese government and peaceful citizens of Lebanon through this crisis and provide the support they need to weather this storm," she added.
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 political movement with a powerful guerrilla army.
The fighting, the 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 pro-Western government had declared war.
A senior State Department official, who spoke on condition of anonymity, said Washington saw the latest conflict as political in nature, but there was a fear it could escalate into a Sunni-Shi'ite war.
He said the State Department was making contingency plans to secure the US Embassy and its personnel and ensuring plans were in place to help US citizens living in Lebanon if the situation deteriorated.
Speaking in Crawford, Texas, where US President George W Bush was at his ranch preparing for his daughter's wedding, Johndroe said: "We ... urge Iran and Syria to stop their support of Hezbollah." Bush has led international campaigns aimed at diplomatically isolating Tehran and Damascus.
Bush is due to meet Siniora on May 18 at the Egyptian Red Sea resort of Sharm el-Sheikh at the end of a week-long visit to the Middle East. - Reuters