Five U.S. troops were killed in two separate roadside bombings on Monday, a military official said.

"We lost five soldiers yesterday in two unfortunate incidents, both involving IEDs," Rear Adm. Gregory Smith, director of the Multi-National Force-Iraq's communications division, told reporters in Baghdad's heavily guarded Green Zone.

In a third incident, American forces said a sailor died of injuries from an explosion Monday in Salahuddin province, north of Baghdad.

The U.S. said it planned to release nine Iranian prisoners in the coming days, including two captured when U.S. troops stormed an Iranian government office in Irbil last January. The office was shut after the raid, but it reopened as an Iranian consulate on Tuesday, Iraqi and Iranian officials said.

Iran appears to have kept its promise to stop the flow into Iraq of bomb-making materials and other weaponry, or EFPs, that Washington says has inflamed insurgent violence and caused many American troop casualties, a military spokesman said.

Defense Secretary Robert Gates said last week that Iran had made such assurances to the Iraqi government.

"It's our best judgment that these particular EFPs ... in recent large cache finds do not appear to have arrived here in Iraq after those pledges were made," Smith said.

The five casualties make 2007 the deadliest year for U.S. soldiers in Iraq, according to the Associated Press.

At least 852 American military personnel have died in Iraq so far this year — the highest annual toll since the war began in March 2003, according to AP figures. Some 850 troops died in 2004.

But the American military in Iraq reached its highest troop levels in Iraq this year — 165,000. Moreover, the military's decision to send soldiers out of large bases and into Iraqi communities means more troops have seen more "contact with enemy forces" than ever before, said Maj. Winfield Danielson, a U.S. military spokesman in Baghdad.

"It's due to the troop surge, which allowed us to go into areas that were previously safe havens for insurgents," Danielson told the AP on Sunday. "Having more soldiers, and having them out in the communities, certainly contributes to our casualties."

The Associated Press contributed to this report.