BAGHDAD: Gunmen killed eight security guards Tuesday during an early morning robbery at a bank in central Baghdad, an attack that appears to be the work of insurgents attempting to finance operations against Iraqi and U.S. forces, police officials said.

It is the second deadly robbery in a week in Baghdad's commercial Karradah district.

The gunmen broke into the state-run Rafidain Bank at about 4 a.m., killing three on-duty guards and five others on the premises, who were either on a break or asleep in beds, said an Iraqi police official.

Investigators believe the gunmen used silencer-fitted weapons during the attack because witnesses and neighbors didn't hear any gunshots.

Another official said the robbery appears to be the work of insurgents trying to get hold of cash to finance their operations. The official did not say what evidence led police to believe the robbery was the work of insurgents.

Both officials spoke on condition of anonymity because they were not authorized to release the information to the media.

Five people were killed Sunday in a daylight attack at a popular money exchange office, a reflection of the increasing crime in Iraq even as violence is on the decline.

The gunmen broke into the al-Nibal money exchange office in downtown Baghdad shortly before noon, killing three employees and two customers, said Iraqi police officials. They said 12 others, including eight employees, were wounded in the attack in Baghdad's commercial Karradah district.

Insurgents in Iraq and Afghanistan are believed to have staged bank robberies, kidnappings for ransom and other criminal activities to raise funds.

Although violence has dropped dramatically over the past two years, the number of robberies in Iraq appears to be increasing.

In April, gunmen killed at least seven people during a daylight heist of jewelry stores in Baghdad. In the same month, gunmen used similar tactics, killing two, during the robbery of a currency exchange in the southern city of Basra.