BAGHDAD, Iraq -- Insurgent car bombs struck a market and a police bus Friday, killing at least 25 people, and a dozen bodies were uncovered in a garbage dump on the outskirts of Baghdad -- some victims blindfolded and shot execution-style.

Also Friday, Iraqi militants holding an Australian engineer hostage issued a 72-hour ultimatum for Australia to start pulling its troops out of Iraq.

The latest insurgent attacks were part of a surge in violence that has killed more than 270 people -- many of them Iraqi soldiers and police -- since Prime Minister Ibrahim al-Jaafari announced his new government April 28.

The bodies, the latest in a series of gruesome finds, were discovered by scavengers sifting through garbage for scrap metal and other items to sell at a dump on Baghdad's northeastern outskirts, police and soldiers said.

The victims, believed to be Iraqis, were found in shallow graves and seemed to have been killed recently, al-Maslokhi said. Some were blindfolded and had been shot in the head, he said.

In nearby Suwayrah, 25 miles south of the capital, a suicide car bombing at a market killed 17 civilians and wounded 46, police, hospital and government officials said.

Such attacks often target U.S. military patrols, Iraqi security forces or mosques, but police said there were no obvious targets Friday.

In Saddam's hometown of Tikrit, a car packed with explosives -- and with a taxi sign on its roof -- destroyed a police minibus, said U.S. Army 1st Sgt. Brian Thomas and Iraqi army Maj. Salman Abdul Wahid.

The attack at a checkpoint on the eastern outskirts of Tikrit, 80 miles north of Baghdad, killed at least eight policemen, said police Lt. Col. Saad Abdul Hamid.

Elsewhere, two insurgents fired at American soldiers on patrol in south Baghdad, and one militant was killed in the return fire, the U.S. military said.

In the holy city of Najaf, 100 miles south of Baghdad, gunfire broke out outside a mosque controlled by followers of radical Shiite cleric Muqtada al-Sadr. Five people were wounded, worshippers and a military official said.

An official in al-Sadr's Najaf office, Fadhel Mohammed, accused Iraqi security forces of firing at worshippers as they left the mosque chanting slogans in support of their leader. Iraqi army Maj. Jassim Mohammed said the worshippers threw stones at the soldiers, who fired in the air to disperse the crowd.

Both sides played down the incident, saying they did not want it to escalate. Al-Sadr's followers launched two major uprisings last year, but have remained quiet since signing agreements with U.S. authorities in August.

Arab television station Al-Jazeera aired new footage of the Australian hostage Douglas Wood, and said the militants holding him gave Australia 72 hours to start withdrawing its forces from Iraq. It did not say what the militants would do if their deadline isn't met.

In the footage, the 63-year-old California resident, who suffers from a serious heart condition, is shown with his head shaven and rifles pointed at him. Australia's government has said it will ignore demands to remove its 1,370 troops.

Al-Jazeera also aired footage Friday purporting to show six Jordanian hostages captured by a different militant group demanding that all Jordanian companies stop cooperating with U.S. forces. The authenticity of the video could not be verified.

More than 200 foreigners have been kidnapped in Iraq since Saddam was ousted in April 2003. Some have been seized for ransom, others have been used to pursue political goals. More than 30 have been killed by their captors.

---

Associated Press photographer Karim Kadim contributed to this report.