A homocide bomber detonated a car bomb near a crowd of people waiting to apply for jobs with the Iraqi National Guard (search) in the northern city of Kirkuk (search) on Saturday, killing at least 20 people and wounding 16, officials said.

Also Saturday, the pan-Arab television station Al-Jazeera (search) aired video footage of what it said were two Americans and a Briton kidnapped two days ago from their house in central Baghdad. Masked gunmen in the footage said they would be killed if female inmates are not released from two Iraqi prisons in 48 hours.

The footage shown on al-Jazeera showed them blindfolded and said they were being held by alleged terror mastermind Abu Musab al-Zarqawi (search). The authenticity of the tape could not immediately be confirmed.

The Kirkuk car bombing was the third this week targeting Iraq's beleaguered security forces. The escalating violence has put new pressure on Iraqis working to restore stability in their country — but seen as collaborators because of their cooperation with U.S. forces.

On Friday, at least 52 people were killed in nationwide violence. Dozens died in a wave of U.S. airstrikes in and around the insurgent stronghold of Fallujah, west of Baghad, targeting fighters loyal to al-Zarqawi.

The street outside the guard headquarters was littered with bloodied bodies, debris and shards of glass. Ambulances with sirens wailing rushed to the site of the explosion and firefighters doused flames leaping up from a burning car.

Kirkuk's National Guard chief, Maj. Gen Anwar Mohammed Amin, said at least 20 died and 16 were wounded in the attack.

"I saw a speeding car crossing an open field heading toward the would-be recruits, then there was a huge explosion and a big fire," said Asu Ahmed, a street vendor. "There were many dead and injured people and I helped put them in ambulances."

Maj. Thomas Williams, as spokesman for the Army's 2nd Brigade, 25th Infantry Division, said there were no American casualties.

Initial reports indicated that the victims were civilians and that no guardsmen were killed or wounded, Williams said. He said guardsmen opened fire as the suicide attacker sped toward the building before the car blew up.

"This a terrorist act and all the victims are young Iraqis who just wanted to make living by joining the Iraqi National Guard," said police Col. Sarhat Qadir.

Saturday's bombing came exactly a week after another suicide attacker detonated a car bomb outside an Iraqi police academy in Kirkuk as hundreds of trainees and civilians were leaving for the day. That attack killed 20 people and wounded 36.

The Iraqi National Guard is the centerpiece of U.S.-backed efforts to build a strong Iraqi security force capable of taking over security operations in many towns and cities from American troops before January elections.

A wave of bombings, mortar attacks and shooting sprees targeting police forces and potential recruits has killed hundreds of people nationwide. On Friday, a suicide attacker rammed an explosives-packed vehicle into a row of police cars in the capital, killing at least three, days after a car bomb near a Baghdad police station left dozens dead.

The Americans insisted Saturday that the militants' campaign of violence won't succeed.

"The continued targeting of Iraqi Security Forces shows the desperation of anti-Iraqi forces as they recognize the continued improvement and capability of the Iraqi National Guard and Iraqi Police," said Maj. Neal O'Brien of the Army's 1st Infantry Division.

In the video aired Saturday, Al-Jazeera's announcer said Tawhid and Jihad, the militant group led by al-Zarqawi, warned that the three would be executed within 48 hours unless the United States and Britain release female Iraqi inmates from Abu Ghraib and Umm Qasr prisons. Umm Qasr prison is in southern Iraq, where British troops are stationed.

Lt. Col. Barry Johnson, a U.S. military spokesman in Baghdad, said coalition forces do not hold any women at Abu Ghraib or at Camp Bucca, a U.S. detention facility near Umm Qasr.

"The only females we hold are two high-value detainees, which are kept with the other approximately 100 high-value detainees in a separate, secure location," Johnson said.

Kidnappers abducted Americans Jack Hensley and Eugene Armstrong and Briton Kenneth Bigley at dawn Thursday from their home on a leafy Baghdad street. The three were doing construction work in Iraq.

Al-Jazeera, a Qatar-based Arab satellite news station, showed only a brief clip without any audio.

An armed militant held his rifle pointing downward at the head of one of the three men, who were blindfolded, seated and appeared to be unharmed. A British diplomat in Baghdad said the Embassy was aware of the tape but would not comment on the report. U.S. Embassy officials were not immediately available.

Earlier in Baghdad, a roadside bomb exploded in a small sidestreet in the city center Saturday, killing one man and seriously wounding two in a passing car, police and witnesses said.

Residents identified the three men as Iraqis who worked as security guards at the nearby al-Sadir Hotel, a heavily fortified complex where U.S. civilian contractors and other Westerners involved in Iraq's reconstruction effort are known to live and work.

The three men were wearing uniforms, body armor and carrying assault rifles, according to the witnesses.

West of Baghdad, residents found the body of Anbar province's deputy governor, who was kidnapped earlier this month, hospital officials said Saturday. Bassem Mohammed's body was found Friday evening near Ramadi General Hospital, said Dr. Abdel Munim Aftan, the facility's director.

Also Saturday, a senior official with Iraq's state-run North Oil Co. survived an assassination attempt when his convoy came under attack in the northeastern city of Mosul, police said.

Mohammed Zibari, the head of the company's oil products department in the Nineveh province, was traveling in a three-vehicle convoy on his way to work in Mosul when unidentified gunmen opened fire, killing five of his bodyguards and wounding four others, said police Lt. Mohammed Ali.

Zibari, who is in charge of supplying the province with oil and gas, escaped unharmed, Ali said. Two cars were badly damaged in the attack.

Insurgents have repeatedly targeted Iraq's crucial oil infrastructure in a bid to destabilize Iraq and undermine the U.S.-backed interim authorities.