Sporadic explosions and gunfire echoed through this holy Shiite city on Saturday after two days of intense clashes between U.S. forces and Shiite Muslim insurgents that marked some of the bloodiest fighting in Iraq (search) in months and killed up to 300 people.

A 24-hour government deadline for the militants to leave Najaf (search) expired Saturday without any sign of a pullout — or any major attack. The city was the quietest it's been since fighting erupted Thursday between American troops and militants loyal to Shiite cleric Muqtada al-Sadr (search) and spread to several cities across Iraq including the capital.

U.S. warplanes flew overhead in Najaf and American armored vehicles and Humvees blocked the main roads into the city, and most streets were deserted. The militants remained defiant.

"If the occupying forces insist on fighting, we will defend the city until the last drop of our blood," said Mushtaq Khafajy, an al-Sadr spokesman.

The Iraqi government says it is determined to crush all militias in the country, including al-Sadr's Mahdi Army, and Najaf Gov. Adnan al-Zurufi on Friday gave insurgents 24 hours to leave the city.

The U.S. military said two Marines were killed in Najaf on Friday "as a result of enemy action." Also Friday, an insurgent fired a rocket-propelled grenade at a patrolling U.S. vehicle in Baghdad, killing one soldier.

In the southern city of Basra (search), gunmen attacked the governor's office at dawn with rifles and mortar rounds. Police returned fire, repelling the attack and killing one of the gunmen, police Capt. Mustaq Talib said.

The violence Thursday and Friday threatened to re-ignite a bloody, two-month Shiite insurrection that broke out in April — and the heavy U.S. response appeared designed to quash militia activity quickly and prevent a repeat.

A renewed Shiite uprising would cause severe problems for Iraq's fledgling interim government as it tries to gain popular support and for coalition forces that are already struggling against Sunni militants.

Clashes Thursday and Friday in Baghdad's Shiite neighborhood of Sadr City killed 35 people and wounded 180, the Health Ministry said. Separate attacks blamed on al-Sadr's followers also wounded 15 American soldiers in Baghdad, the U.S. military said.

Casualty figures for Najaf varied widely. The U.S. military said 300 militants were killed in Najaf, but Ahmed al-Shaibany, an al-Sadr aide in Najaf, said only nine militants were killed and 20 injured. Hussein Hady, an administrator at Najaf's main hospital said at least 19 civilians were killed and 68 others wounded.

Two U.S. Marines and an American soldier were also killed in Najaf on Thursday, and 12 troops were wounded, the military said.

Friday's clashes were the fiercest since the fall of Saddam Hussein, with U.S. helicopter gunships and fighter jets pounding insurgents hiding in a sprawling cemetery in Najaf.

Al-Sadr blamed the violence on the United States, which he called "our enemy and the enemy of the people," in a sermon read on his behalf at the Kufa Mosque near Najaf.

The heavy battles came as the most powerful Shiite cleric in Iraq, Grand Ayatollah Ali Husseini al-Sistani, arrived Friday in Britain to receive medical treatment for what an aide called "a health crisis" involving his heart.

The 73-year-old ayatollah, who holds enormous influence among Iraq's Shiite majority, has played a largely moderating role, urging Shiites not to resort to anti-U.S. violence, and during al-Sadr's first uprising he played a role in trying to calm the crisis.

There was no information on the seriousness of al-Sistani's condition, but the trip was his first time out of Iraq in years. The aide, Sheik Hamed Khafaf, said al-Sistani "needs special treatment, but he is not in a deteriorated state."

Al-Sadr aides called for a return to the truces that have kept relative calm for the past two months, and other Shiite leaders were trying to restore a cease-fire.

The Mahdi Army has proved difficult to put down in the past. It persisted despite heavy casualties during its first uprising, and U.S. commanders — hesitant to carry out a full-fledged assault in the holiest Shiite city — were forced to back down from vows to uproot the militia.

In the capital, guerrillas fired five mortar rounds into central Baghdad about 7:30 a.m. Saturday, damaging two sport utility vehicles, but causing no other casualties or serious damage, the U.S. military said.

Two other mortar shells exploded near the Iranian Embassy just before midday, wounding five people, a police officer said on condition of anonymity.