Kidnappers holding four Christian peace activists gave U.S. and Iraqi authorities a "last chance" to release all detainees in Iraq, threatening to kill the hostages if their demands were not met in a videotape broadcast Saturday.

At least 22 people were killed in scattered violence across the country, including a U.S. soldier in a roadside bombing in Baghdad and 10 Iraqis in a bombing at a candy store.

The hostages — two Canadians, an American and a Briton — were shown on a tape broadcast by Al-Jazeera looking gaunt and standing near a white wall in what appeared to be a house.

The pan-Arab station's announcer said the hostage-takers, the "Swords of Righteousness Brigades," issued a statement warning of the "last chance" for U.S. and Iraqi authorities to "release all Iraqi prisoners in return of freeing the hostages."

"Otherwise, their fate will be death," the statement added, without mentioning a deadline.

The broadcast of the Jan. 21-dated video capped a week in which two German engineers were abducted in the northern industrial city of Beiji and the U.S. military released five Iraqi women who had been in military custody — a move demanded by the kidnappers to spare her life of American reporter Jill Carroll. The military said the prisoner release was routine and not in response to the ultimatum.

Meanwhile, an influential Shiite official said Saturday that Iraq's religious Shiite bloc will insist on control of the state security forces in negotiations to form a new, broad-based government.

The issue of which group controls the defense and interior ministries is expected to be a major obstacle to forming a new government with greater Sunni Arab representation, a key U.S. goal as talks get underway after last month's national election.

Sunni Arabs accuse Interior Ministry commandos of torturing, abusing and indiscriminately detaining civilians in the name of fighting the Sunni insurgency.

Shiite politicians maintain that they need control of the security services to prevent a repeat of the oppression their community suffered under Saddam Hussein's Sunni-dominated regime. Sunni politicians, by contrast, have complained of torture, abuse and arbitrary arrest by Shiite-controlled Interior Ministry forces.

A Sunni Arab political leader criticized a Friday police crackdown on Sunni neighborhoods in southern Baghdad, which saw about 60 people detained and three killed, apparently by insurgents.

"We condemn the treacherous and terrorist acts that have targeted and killed dozens of innocent people who were only guilty of rejecting the (U.S.-led) occupation," Khalaf al-Ilyan said at a press conference.

Sunni Arabs, who lost power after Saddam's ouster, are the insurgency's driving force and the U.S. wants them brought into the political process to blunt the violence.

Police said the bound, gagged and shot bodies of two men in their 40s were found in southern Baghdad's Rustamiyah sewage plant, which al-Ilyan said had "become the place where families go to search for the bodies of their sons killed by the government forces or militias."

Police also found the buried bodies of six laborers who had been bound, gagged and shot in the head near the southern city of Karbala, spokesman Rahman Mashawi said.

Hadi al-Amri, head of the Badr Organization militia and a key figure in the Supreme Council for the Islamic Revolution in Iraq, the biggest Shiite party, said Shiites would not relinquish control of the defense and interior ministries.

"We have red lines that cannot be crossed ... we will never surrender these," he said. "We are subjected to a daily slaughter. We will not relinquish security portfolios."

Relatives and co-workers Saturday welcomed the footage of Canadians James Loney, 41, and Harmeet Singh Sooden, 32; Tom Fox, 54, of Clear Brook, Va., and Norman Kember, 74, of London. The four were seized Nov. 26 as they were working with Christian Peacemaker Teams, which investigates allegations of abuse against Iraqi prisoners.

"We're still very concerned, but at least we have proof that they are alive," Loney's youngest brother, Matthew, said in a telephone interview from Vancouver, British Columbia. "The four look to be OK, but it looks like they have lost weight."

The video, which could not be independently authenticated, was the third released showing the four activists in captivity.

Al-Jazeera editor Saad al-Dosari declined to say how the station obtained the tape, which was about 55 seconds long and aired in its entirety.

Christian Peacemaker Teams, which has offices in Toronto and Chicago and has been working in Iraq since October 2002, said it was encouraged to see the activists alive and called on their kidnappers to release them unharmed.

"This news is an answer to our prayers. We continue to hope and pray for their release," Rebecca Johnson, a Canadian-based coordinator for the group, said.

More than 250 foreigners have been taken hostage in Iraq since Saddam's ouster, and at least 39 have been killed.

In continuing violence around the country, a U.S. soldier was killed in central Baghdad, where gunmen also shot to death a prominent professor and political analyst who often appeared on Arab TV talk shows to discuss Iraqi politics, police said. During a recent appearance on a panel show, Abdul-Razzaq al-Naas, a Shiite, spoke out strongly against the government's failure to improve security and the economy.

In Iskandariyah, 30 miles south of Baghdad, a bomb hit a candy store in a market Saturday evening, killing 10 people and wounding three, police Capt. Muthanna Khalid said.

At least seven other people were killed in separate shootings across Baghdad, police said, and two policeman were killed in a shooting and a bombing in other areas.

A Marine also was killed Friday in a non-hostile vehicle accident in Fallujah, the military said. At least 2,240 members of the U.S. military have died since the Iraq war started in March 2003, according to an Associated Press count.