BAGHDAD, (Reuters) – Attacks on U.S. and Iraqi forces in Baghdad increased sharply in March, the U.S. military said on Tuesday, and the U.S. ambassador warned violence would spiral further if U.S. troops were withdrawn too soon.

For a third day running, fighting between Iraqi troops and gunmen erupted in Sadr City, the slum in eastern Baghdad that is a stronghold of radical Shi’ite cleric Moqtada al-Sadr. Police said eight people had been wounded.

The figures reflecting a sharp increase in fighting at the end of March, between security forces and Sadr’s black-masked Mehdi Army militia, emerged hours before top U.S. officials in Iraq are due to give a progress report to Congress.

The renewed violence could embolden Democrats who argue that the year-long “surge”, which boosted U.S. troop numbers in Iraq by about 30,000, has been a failure and call for troop withdrawals soon.

The Republican administration says overall violence levels in Iraq are still down dramatically from a year ago and sectarian attacks on civilians remain sharply lower.

U.S. military spokesman Lieutenant-Colonel Steven Stover told Reuters on Tuesday that overall attacks in the Iraqi capital had averaged 23 per day last month, up sharply from an average of nine a day over the November-February period.

Attacks on civilians declined steadily through March, meaning virtually the entire increase was attributable to attacks on U.S. and Iraqi troops.

In April, attacks in Baghdad eased back to an average of 11 per day, Stover said. But that average does not include a sharp rise in violence since Sunday, when Sadr’s Mehdi Army militia resumed rocket and mortar attacks on the Green Zone, the heavily fortified compound in central Baghdad which is home to U.S. forces and the Iraqi government.

Deaths of U.S. soldiers averaged a little more than one per day over the past four months, but eleven have been killed over the past two days.

Ahead of his congressional testimony on Tuesday, U.S. Ambassador to Iraq Ryan Crocker told CNN television that a premature withdrawal of U.S. troops would lead to violence not seen since the U.S.-led invasion to oust Saddam Hussein in 2003.

“As bad as it was in 2006 we were here. If we spiral into conflict again and we’re leaving, everybody knows we’re not coming back so I think the gloves then come completely off,” Crocker said. “It’s in that environment that the risk of regional involvement in the conflict, particularly from Iran, becomes very grave indeed,” he added.

The United States currently has about 158,000 troops in Iraq and plans to pull out five brigades, or roughly 20,000 soldiers, by July under plans announced last year.

General David Petraeus, the top U.S. commander in Iraq who is due to testify alongside Crocker, is expected to argue against committing to further withdrawals in 2008.

A “pause” in troop reductions is likely to rile Democrats and other opponents of the Bush administration’s Iraq policy.

On Monday, Iraqi Prime Minister Nuri al-Maliki, who launched a crackdown on Sadr’s Mehdi Army late last month which ended months of relative calm, ordered the cleric to disband his militia or face exclusion from the Iraqi political process.

Aides to Sadr said he was ready to dissolve his fierce fighting force but only if Shi’ite religious leaders ordered it.

Sadr’s Mehdi Army has been a major actor throughout Iraq’s five-year-old war and the main foe of U.S. and Iraqi forces in battles in Baghdad and across southern Iraq in recent weeks.

Maliki ordered a crackdown on the militia two weeks ago in the southern city of Basra, leading to the country’s worst fighting since at least the first half of 2007.

Fighting continued in Baghdad on Tuesday, Stover said.

Near the town of Baquba, the capital of Diyala province northeast of Baghdad, a roadside bomb struck a minibus killing six people, including four children and two women, and wounding five, police said.