Baghdad, March 24 (Xinhua) The US military said Monday a roadside bomb killed four servicemen Sunday night, pushing the total number of US soldiers killed in action in Iraq to 4,000, days after the fifth anniversary of the war.

The military said in a statement that the four died when their vehicle came under a roadside bombing attack in southern Baghdad. One soldier was injured in the incident.

The new casualty figure came to light amid heated debate in the US presidential election campaign on whether the troops should be pulled out soon.

US President George W. Bush Wednesday defended his Iraqi policy, vowing no retreat. He promised American soldiers would triumph despite the 'high cost in lives and treasure'.

Hillary Clinton and Barack Obama, both vying for the Democratic presidential candidature, pledge to end the war, if elected.

John McCain, who is the Republican candidate, supports Bush's stance.

Roadside bombs have been the top killers of the US troops in the Iraq war, resulting in about 40 percent of the total number of deaths, according to Icasualties, a website which traces coalition casualties.

The US military suffered the largest loss of 901 lives in 2007, when the death toll surpassed 100 for three consecutive months.

The deadliest month was November 2004 when 137 American troops died, according to the website. The loss was sustained during the offensive on the then insurgent hotbed of Anbar province.

Anbar, which has become an example of security and peace following local resistance against the Al Qaeda, also saw 1,282 US troops killed. In Baghdad the US casualties were 1,255 and in the northern province of Salaheddin, 376.

The US plans to pull out five brigades by summer, leaving behind about 140,000 troops. The military is cautious about further drawdown, saying a period of time is needed to consolidate the security gain and evaluate the situation.

The US military maintains that Al Qaeda is still capable of making a comeback.

The website said about 29,000 US troops have been injured.

It said Britain, which has the second largest military presence of about 5,000, has lost 175 soldiers.