US forces suffered one of their costliest days in Iraq yesterday when 19 troops were killed, including 12 on a helicopter and five in a clash in a Shia holy city that the US military blamed on militiamen.
The deaths come as a brigade of around 3,200 extra US soldiers arrived today in Baghdad, the first of some 17,000 planned reinforcements for the city.
The US military said the troops would be fully operational by the start of February. Meanwhile several bombs kept up the pressure today.
One killed six people on a bus in the centre of Baghdad while a car bomb killed one more in the capital. In the southern city of Basra, a roadside bomb killed a British soldier and wounded four more.
The battle at a government building in Kerbala was the bloodiest for US troops in the Shia south in two years and occurred as President George W. Bush presses leaders of the Shia majority to crack down on militias from their community.
The US military today revised the number killed in yesterday's Blackhawk helicopter crash to 12, including four crew. The US military said another five soldiers were killed and three wounded in the Kerbala clash.
Two other soldiers were killed elsewhere, and the deaths of two killed on Friday were also announced. It was unclear whether the helicopter was shot down.
Residents near Baquba in violent Diyala province northeast of Baghdad said they saw a helicopter in flames in the air. It was the deadliest day for US forces since Mr Bush announced he was sending about 20,000 more troops to Iraq to try to prevent sectarian civil war between Shia Muslims and the once-dominant Sunni Arab minority.
His plans have run into resistance from opposition Democrats who now control Congress. The US military said today a brigade of around 3,200 soldiers had arrived in Baghdad, the first of some 17,000 planned reinforcements for the city, and it would be fully operational by the start of February.
Thousands of pilgrims are thronging Kerbala, 110 km (70 miles) south of Baghdad, at the start of the 10-day rite of Ashura, a highpoint of the Shia calendar and a target before for attacks by al-Qaeda and other Sunnis.
"The Provincial Joint Coordination Center (PJCC) in Kerbala was attacked with grenades, small arms and indirect fires by an illegally armed militia group," the US military said in a statement, apparently blaming Shia militiamen rather than Sunni insurgents whom it usually refers to as "terrorists".
"Five US soldiers were killed and three wounded while repelling the attack." It made no mention of attackers killed or detained and officials did not immediately respond to queries on how the assailants' identity was established. "We do know they're militiamen but we're not going to go any further than that," US Major Steven Lamb said today, adding that investigations were continuing.
Relations have become strained between Washington and the Shia-led Iraqi government as the United States presses Baghdad to rein in Shia militias blamed for death squad killings and tries to limit the influence of Shia Iran.
Not since US troops fought street battles with the Mehdi Army of Shia cleric Moqtada al-Sadr in southern Iraq in 2004 have US forces had such heavy casualties in the region.
Meanwhile the political movement of al-Sadr said it would end a two-month boycott of parliament today, smoothing over a rift with its Shia allies in the US-backed government.
Shia Prime Minister Nuri al-Maliki has been under pressure to crack down on the Mehdi Army, a militia loyal to Sadr that the United States sees as the biggest threat to security in Iraq.
But his past dependence on Sadr's political support has made that difficult.
The Sadrists announced a boycott late last year to press their demand for a timetable for the withdrawal of US troops from Iraq and to protest against a meeting between Mr Maliki and US President George W. Bush.
"We are ending our boycott of the ministries and the parliament," Mr Bahaa al-Araji, a senior member of the Sadrist group, told a news conference with the ruling Shia Alliance, adding that there had been a response to their demands.