A series of explosions rocked central Baghdad Monday night and witnesses reported seeing smoke rising from the heavily fortified Green Zone. The U.S. military said it had no immediate information on the blasts.

About a dozen blasts began about 10 p.m. and lasted about five minutes.

Iraqi police said several mortar shells landed in the Green Zone, which houses the U.S. and British embassies, the Iraqi government headquarters and thousands of American troops on the west bank of the Tigris River.

Sirens could be heard from the area.

Insurgents and militia fighters routinely fire rockets and mortars into the Green Zone. The attacks seldom cause casualties or damage because they are poorly aimed and the 4-square-mile zone contains much open space. But two Americans — a contractor and a soldier — were killed in late March in a rocket attack on the area.

Also Monday, the U.S. military said five U.S. troops were killed in separate attacks this weekend, including three in a single roadside bombing in Baghdad.

Two attacks occurred in eastern Baghdad, a predominantly Shiite area where American and Iraqi forces have stepped up their activities as part of a security crackdown that began on Feb. 14 to quell the sectarian violence. A Marine also was killed in Anbar province, a Sunni insurgent stronghold west of the capital.

In other violence, a homicide car bomber apparently targeting an Interior Ministry convoy struck an Iraqi checkpoint near a busy square in the predominantly Sunni area of Harthiyah in western Baghdad, killing four people and wounding 10, police said.

The bomber detonated his payload, causing part of the road to buckle, as he emerged from an underpass and was heading toward the checkpoint being manned by Interior Ministry commandos. Those killed included two commandos and two civilians.

The violence occurred despite stringent security measures during the security crackdown now in its 11th week.

On Sunday, Iran agreed to join the U.S. and other countries at a conference on Iraq this week, raising hopes the government in Tehran would help stabilize its violent neighbor and stem the flow of guns and bombs over the border.

Senior Iranian envoy Ali Larijani flew to Baghdad on Sunday for talks with al-Maliki and other senior Iraqi officials ahead of this week's meetings in Egypt — the highest-ranking Iranian official to visit Iraq since the collapse of Saddam Hussein's regime in 2003.

Earlier this month, U.S. military spokesman Maj. Gen. William Caldwell said Iranian intelligence operatives have been training Iraqi fighters inside Iran on how to use and assemble deadly roadside bombs known as EFPs, or explosively formed penetrators.

He said Iranian support extended to Sunnis as well as Shiites in Iraq, showing reporters photographs of what he said were Iranian-made mortar rounds, RPG rounds and rockets that were found recently in a Sunni neighborhood of Baghdad.

The killings of the Americans came as U.S. troops have been increasingly deployed on the streets of Baghdad and housed with Iraqi troops in joint security stations away from their heavily fortified bases, raising their vulnerability to attacks.

The roadside bomb killed three soldiers and wounded another while they were on a combat patrol Sunday in eastern Baghdad, the military said, without providing more details. An Iraqi interpreter also was killed in the attack.

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The deaths raised to at least 104 American troops who have died in Iraq as April draws to a close. The U.S. monthly death toll has topped 100 five other times since the Iraq war began in March 2003, according to an Associated Press count based on military figures.

At least 3,351 members of the U.S. military have died since the war started, according to the AP count.

Another soldier was killed by small arms fire in eastern Baghdad on Saturday, and a Marine was killed in fighting Sunday in Anbar, according to separate statements.

In northern Iraq, a parked car bomb struck a police patrol in the Raas al-Jada, a mainly Sunni Arab area in the northern city of Mosul, killing one policeman and wounding two others, police Brig. Gen. Mohammed Idan al-Jubouri said.

The attack occurred at 8 a.m., about four hours after some 50 gunmen attacked a police station in the same area, prompting a firefight and clashes as police chased the gunmen through the narrow streets. Four of the gunmen were killed and two others detained, while one policeman was wounded, police said.

Police also cordoned off the area and blocked five bridges after four mortar rounds landed on the police command headquarters elsewhere in Mosul, causing no damages, said Brig. Saeed Ahmed al-Jubouri, the media director for the provincial police.

Complete coverage is available in FOXNews.com's Iraq Center.