BAGHDAD, March 23 (UPI) -- Four U.S. soldiers died Sunday when a bomb exploded near their vehicle in Baghdad, bringing the number of U.S. military deaths in the Iraq war to 4,000.

The deaths occurred as attacks against U.S. forces had been declining and coalition officials had been claiming substantial improvement in conditions in Iraq, the Los Angeles Times reported.

The attacks came on a day in which at last 60 Iraqis were killed in firefights, bombings and mortar and rocket attacks across Iraq.

The carnage hit both Shiite and Sunni communities in central and northern sections of the country. Baghdad's fortified Green Zone was hit by mortar and rocket fire but there were no immediate reports of casualties there, the newspaper said.

The deadliest incident involved a suicide bomber who detonated an explosives-packed tanker outside an Iraqi army base in Mosul, killing at least 12 Iraqi soldiers and wounding 30 other people, said Brig. Gen. Mohammed Ahmed.

A second car bomber hit an Iraqi army checkpoint in Mosul, killing one officer and injuring 10 others.

Police in Zafaraniya said insurgents in three cars shot commuters waiting to board minibuses, killing seven and injuring 16.

Another suicide bomber in Shula district killed seven people and injured 12 more as they waited in line to buy gasoline.

In Baghdad, police found the bodies of six people killed execution-style.

Other attacks left two police officers dead and two injured in Diyala province, four soldiers died in a roadside blast south of Kirkuk and U.S-led forces reported killing at least 12 suspected insurgents in a firefight east of Baqubah, the Times reported.