Three suicide car bombs, including one that exploded near the Iraqi prime minister's party headquarters in Baghdad, and a roadside explosion killed at least 16 people Monday as insurgents pressed their deadly campaign to disrupt national elections, which Iraq's defense minister said could be postponed to give the Sunni minority time to participate.

A car bomb exploded late Monday near a U.S.-manned checkpoint to the Green Zone (search), the heavily fortified area housing the U.S. Embassy and Iraqi government offices, U.S. Embassy spokesman Bob Callahan said. U.S. troops surrounded a burning sport utility vehicle at the scene.

Three bodies were seen burning inside the destroyed vehicle. The nationalities of the victims, identified as employees of the U.S.-based risk consulting group Kroll Inc., (search) were not immediately known.

Iraqi police Lt. Khalid Mohammed said the bomb targeted a U.S. civilian convoy and there were casualties at the checkpoint, which is the main Green Zone exit for trips to Baghdad International Airport (search) west of the city. American contractors and diplomats commonly make the journey along the dangerous airport road in SUVs.

Meanwhile, Iraqi Defense Minister Hazem Shaalan raised the possibility that elections scheduled for Jan. 30 could be postponed to try to persuade minority Sunni Muslims to participate in the vote.

The first strike by an explosives-laden car near interim Prime Minister Ayad Allawi's Iraqi National Accord (search) party headquarters killed two police officers and one civilian and injured 25 other people. The secular Shiite leader was not inside the building at the time, aides said.

Monday's second attack took place in Balad, about 50 miles north of the capital, killing four Iraqi National Guardsmen and wounding 14, U.S. military spokesman Neal E. O'Brien said. The driver of the car bomb died in the blast.

The third strike occurred in Saddam Hussein's hometown of Tikrit, killing at least six Guardsmen and injuring four others in a roadside explosion, police said.

"Anti-Iraqi forces continue to target the Iraqi National Guard" because the ING is creating conditions for "successful elections," O'Brien said. "Every day they (the ING) get stronger and take more responsibility for security in Iraq."

Car bombings have become a standard feature of the deadly insurgency in Iraq ahead of the elections scheduled for Jan. 30. On Sunday, a car bombing in Balad killed at least 22 National Guardsmen and their bus driver. Ten other people were killed in separate attacks.

U.S. officials warned of violence ahead of the landmark vote for a constitutional assembly, and the guerrillas have made good on those fears with tragic ease. Iraq's poorly equipped security forces usually have far less training than American troops, and attacks on them usually result in more casualties.

The first explosion Monday occurred at about 10 a.m. outside the headquarters of Allawi's Iraqi National Accord party in western Baghdad. Police said a yellow car exploded shortly after trying to ram a police checkpoint.

The blast killed the driver, two policemen and a civilian, and wounded 18 officers and seven civilians. Witnesses said machine-gun fire broke out after the explosion, which set fire to three police vehicles.

The radical Ansar al-Sunnah Army (search) — known for numerous deadly attacks against U.S. troops, Iraqi forces and politicians — claimed responsibility.

"One of Islam's lions managed to carry out a heroic martyrdom operation targeting a large bunch of Iraqi police agents responsible for guarding the headquarters of the National Accord of the apostate Allawi," a statement on the group's Web site said.

In other violence, an Iraqi policeman was killed and two others were wounded when a beheaded, booby-trapped corpse exploded in Mosul as police were trying to identify the body, a government statement said Monday.

"This is another example of how the criminals and terrorists — attempting to thwart Iraq's efforts to conduct free and fair elections — have no regard for their fellow countrymen," the government said.

On Sunday, prominent Shiite leaders belonging to the Unified Iraqi Alliance (search) — a mainstream Shiite coalition running in the election — called for unity with Sunni Arabs wanting to delay the vote but insisted it be held despite the violence.

Shiites, who make up about 60 percent of Iraq's 26 million people, are eager for the vote to go ahead so they can take power long denied them when the Sunni Arab minority had power under Saddam Hussein. But they hope the Sunnis, who make up about 20 percent of the people, will participate so the vote can be considered legitimate.

Iraq's insurgents, believed to be predominantly Sunni, repeatedly have targeted Shiites in apparent attempts to widen sectarian rifts.

Shaalan said during a visit to Cairo, Egypt, that he asked Egypt to try to persuade Sunni Muslims to participate in the vote.

"We could postpone the date to let all Iraqis go to the polls in one day" if that would accommodate Sunnis, Shaalan said.

Other Iraqi and U.S. officials, including President Bush, have insisted the vote will be held as planned. Shaalan is known for taking an independent line, at one point prompting Allawi to publicly distance his interim government from Shaalan's statements.

The Shiite leaders, who are backed by Iraq's most influential cleric, Grand Ayatollah Ali al-Sistani (search), said postponing the vote would only create more chaos. They rejected comments purportedly made by Usama bin Laden in a tape released Dec. 27 in which the Al Qaeda leader urged Muslims not to vote, calling the election illegitimate.

A spokesman for the Independent Electoral Commission in Iraq, Fareed Ayar, refused to comment on Shaalan's statements, saying the body was functioning according to the electoral schedule.

"The commission is still working on holding the elections as scheduled and according to the timetable we have," Ayar said.

A low turnout because of violence or Sunni concerns about being disenfranchised could undermine the legitimacy of the first free elections in Iraq since the monarchy was overthrown in 1958.