Baghdad ? Sunni Muslim insurgents blew up five car bombs and fired mortars into Baghdad’s largest Shiite district Thursday, killing at least 161 people and wounding 257 in a dramatic attack that sent the U.S. ambassador racing to meet with Iraqi leaders in an effort to contain the growing sectarian war.

Shiite mortar teams quickly retaliated, firing 10 shells at Sunni Islam’s most important shrine in Baghdad, badly damaging the Abu Hanifa mosque and killing one person. Eight more rounds slammed down near the offices of the Association of Muslim Scholars, the top Sunni Muslim organization in Iraq, setting nearby houses on fire.

Two other mortar barrages on Sunni neighborhoods in west Baghdad killed nine and wounded 21, police said late Thursday.

The bloodshed underlined the impotence of the Iraqi army and police to quell determined sectarian extremists at a time when the Bush administration appears to be considering a move to accelerate the hand-over of security responsibilities. President Bush plans to visit the region next week to discuss the security situation with Iraqi Prime Minister Nouri al-Maliki.

“We condemn such acts of senseless violence that are clearly aimed at undermining the Iraqi people’s hopes for a peaceful and stable Iraq,” said Jeanie Mamo, a White House spokeswoman.

Iraq’s government imposed a curfew in the capital and also closed the international airport. The transport ministry then took the highly unusual step of closing the airport and docks in the southern city of Basra, the country’s main outlet to the vital shipping lanes in the Persian Gulf.

Iraqis walk past the site of a car bomb explosion in the Sadr City district of Baghdad, Iraq. In the deadliest attack since the beginning of the Iraq war, suspected Sunni-Arab militants used several suicide car bombs and mortar rounds Thursday to kill at least 161 people and wound 257, police said.

Leaders from Iraq’s Shiite, Sunni and Kurdish communities issued a televised appeal for calm after a hastily organized meeting with U.S. Ambassador Zalmay Khalilzad. The U.S. Embassy said it had nothing to report about the session.

Al-Maliki, a Shiite, also went on state TV and blamed Sunni radicals and followers of Saddam Hussein for the attacks on Sadr City – the deadliest on a sectarian enclave since the war began.

The coordinated car bombings – three by suicide drivers and two of parked cars – billowed black smoke up into clouds hanging low over blood-smeared streets jammed with twisted and charred cars and buses.

Hospital corridors and waiting rooms were awash in blood and mangled survivors of bombs that struck at 15-minute intervals in the sprawling Shiite slum, which is a stronghold of the Mahdi Army militia of radical anti-American cleric Muqtada al-Sadr, a key al-Maliki backer.

The militia and associated death squads are believed responsible for the slayings of hundreds of Sunnis since suspected al-Qaida in Iraq militants bombed a revered Shiite shrine in Samarra last February.

That attack set off a surge of retaliatory killings between Shiites and Sunnis that have raged all year.