Updated

Shells slamming into tin-roof homes killed at least 16 civilians in Liberia's war-battered capital and the U.S. Ambassador appealed to rebels to lift their bloody eight-day siege of Monrovia (search) to allow food and aid into the city.

Rebels and government forces on Sunday battled at key crossings leading toward President Charles Taylor (search)'s downtown stronghold, with the insurgents showing no signs of slackening their drive.

The insurgents overnight bypassed one of three embattled spans, Stockton Bridge, leading from the rebel-held island port to mainland Monrovia, government field officers told The Associated Press.

After a night of combat, the rebels were in what had been a government-controlled suburb around the bridge -- claiming to be in control.

"This morning we saw fighters coming in, telling us not to panic," one woman, reached by telephone, said in the New Georgia neighborhood, gunfire and booms echoing behind her. "I'm scared."

Taking northern neighborhoods around the bridge would give rebels a foothold on the mainland, from which they could battle their way toward the government-held downtown.

The rebels are pressing a two-month campaign to take Monrovia, a cut-off, bloodied, disease-ridden and hungry city of at least 1.3 million residents and refugees. Their goal is to drive out Taylor, a former warlord behind nearly 14 years of ruinous conflict in the once prosperous West African nation.

But U.S. Ambassador John Blaney (search) appealed to insurgents to lift their eight-day siege and withdraw, saying "If they want to get to a post-Taylor era, this is the way to do it."

The main rebel movement "needs to show that they have regard for the people of Liberia, that it is not indifferent to the great human suffering that is taking place here," Blaney said at the heavily guarded, high-walled U.S. Embassy.

Taylor claimed Saturday that as many as 1,000 people have died since rebels launched their third attempt in two months to take the capital. Aid workers place the week's toll at about 400.

New bombardments Sunday crashed into neighborhoods. One round landed on a tin-roofed shack near Old Bridge, killing four, according to aid workers. Another fell on a nearby house, killing two.

A shell that had fallen close by late Saturday killed an entire family -- eight adults, two children and the pet dog, aid workers said.

Government and rebel officials have traded blame for the bloody shelling. Witnesses claimed Sunday that some mortars fired by the government were landing short of their rebel target, killing residents on their own side.

Blaney urged the Liberians United for Reconciliation and Democracy movement to pull back to the Po River, six miles outside the capital.

The withdrawal would open up the port and camps outside the refugee-choked city for vitally needed food and aid. Blaney said Taylor had agreed to the proposal.

Sekou Conneh, the rebels' civilian chairman, said they would retreat only when peacekeepers were in place.

"We don't want to hand over the port to Charles Taylor," he said.

Insurgents of the smaller Movement for Democracy in Liberia have advanced to within 30 miles of Buchanan, Liberia's second-largest city and home to a key port.

Rebels now control at least 60 percent of Liberia, a country founded by freed by American slaves in the 19th century.

Under international pressure to intervene, President Bush has ordered U.S. ships to take up positions off Liberia, ready to support a yet-to-materialize West African-led peace force.

Bush has demanded that Taylor, indicted for war crimes in neighboring Sierra Leone, step down.

Taylor says he will do so, accepting an asylum offer in Nigeria, only when peacekeepers arrive.

U.S. Deputy Defense Secretary Paul Wolfowitz reiterated Sunday that the American forces "are going in when there is a cease-fire, when Charles Taylor is leaving, has left."

Wolfowitz told "Fox News Sunday" that the U.S. role is to assist the United Nations and West African countries "to stabilize the situation, to avert a humanitarian disaster.

Outside Rome, Pope John Paul II appealed Sunday for Liberia's warring sides to cease fire to allow in humanitarian aid.