BEIRUT, Lebanon - Unchallenged by Lebanon's army, the Iranian-backed Hezbollah routed Sunnis loyal to the U.S.-allied government and seized control of large swaths of Beirut's Muslim sector Friday in a telling demonstration of its military prowess.

The Shiite fighters' success in three days of street fighting dramatically strengthened the hand of the Hezbollah-led opposition in the bitter political struggle with pro-Western factions over who will guide the country.

But Hezbollah leaders signaled they weren't looking for a bloody showdown by pulling back their fighters late in the day. The group, and gunmen from allied groups, also steered clear of government buildings and made no attempt to advance toward Beirut's Christian area.

The Western-backed government, which holds only a small majority in parliament, and opposition parties led by Hezbollah have been deadlocked for 17 months over the government's course.

Sporadic street clashes had broken out the past year. But combat erupted this week after the Cabinet sought to rein in Hezbollah by ordering the removal of an airport security chief with ties to the group and demanding the dismantling of the movement's private phone network.

The quick humiliation of Sunni fighters - who are far less organized than Hezbollah's militia - showed the Shiite group is more than strong enough to prevent actions it opposes.

At one point Friday, about 100 Hezbollah militants wearing matching camouflage uniforms and carrying assault rifles marched down the capital's main commercial street in a display of might meant to show Prime Minister Fuad Saniora's government who really is in charge.

The army largely stood aside as Shiite fighters scattered their opponents in street battles across the capital's Muslim sector. Lebanon's generals have stayed out of the conflict in fear that intervening could splinter the army along sectarian lines, as happened in the devastating 1975-90 civil war that killed 150,000 people and lefts parts of Beirut a moonscape.

At least 15 people were reported killed since Wednesday - the worst sectarian bloodshed since the civil war.

White House spokesman Gordon Johndroe blamed Hezbollah and its backers for the violence. "The Hezbollah terrorist organization, aided by its Iranian and Syrian sponsors, continues to undermine Lebanon's sovereignty and democratic institutions," he said.

Johndroe said the U.S. was consulting with other governments in the region and with the U.N. Security Council about measures to hold those responsible for the fighting accountable.

The rout of Sunnis was a blow for Washington, which has long considered Hezbollah a terrorist group and condemns its ties to Syria and Iran. The Bush administration has been a strong supporter of Saniora's government and its army the last three years.

Hezbollah's show of military power was certain to both strengthen its own political position and deeply worry a Middle East and Western world that are nervous about Iran's growing influence and its intentions in the region.

"The government tried to show force by shutting down Hezbollah. Hezbollah showed force by pushing back the government," said Jon Alterman, director of the Center for Strategic and International Studies Middle East Program in Washington.

"Hezbollah emerged stronger and the government emerged weaker," Alterman added.

The group's fighters took up positions on street corners and along sidewalks, flagging down the few cars that ventured out on nearly deserted roads to search their trunks.

Nearby, dozens of gunmen from an allied party appeared, some covering their faces with masks and carrying rocket-propelled grenade launchers. Elsewhere, Hezbollah-allied militiamen from the Syrian Social Nationalist Party drove in cars, firing in the air in celebration.

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