The number of US military deaths in Iraq since the invasion began in 2003 has reached 4,000, the military said, after a roadside bomb killed four US soldiers in Baghdad yesterday.

The statement by US-led coalition forces said one US soldier was wounded in the attack.

The US soldiers were killed around 10:00pm local time while on patrol in vehicles in southern Baghdad.

The deaths came on a day when Baghdad's fortified "Green Zone" was assaulted repeatedly by rocket or mortar attack, and police said up to 17 people had been killed by rockets falling short outside the government and diplomatic compound.

The attacks were part of a wider increase in violence in the capital and in the northern city of Mosul, underlining warnings by US military commanders that recent security gains in Iraq are both fragile and reversible.

In the past, the US military has blamed such attacks on the Green Zone on rogue elements of Shiite cleric Moqtada al-Sadr's Mehdi Army militia.

Cleric Sadr has imposed a cease-fire on the militia, but there have been signs that it is fraying.

In Mosul, a suicide truck bomber killed 15 Iraqi soldiers and wounded 45 people, including civilians, in an attack on an Iraqi army base, the Interior Ministry said.

US commanders describe Mosul as Al Qaeda's last urban stronghold in Iraq.

- Reuters