Aziz, the first defense witness for Saddam at his trial for crimes against humanity, was once the international public face of the toppled leader's regime and one of his closest aides.

Aziz, whose family earlier this year said he was seriously ill, looked tired as he testified in the courtroom in Baghdad's heavily-fortified Green Zone where Saddam and his seven co- defendants are standing trial. Aziz is not among those accused.

Seeking to turn the tables in the trial that got under way in October, Aziz said current government officials should be in the dock instead, accusing one of the now ruling Shi'ite parties of having tried to kill him and Saddam in the 1980s.

"I was exposed to an assassination attempt by a political party," he told the court. "I am a victim of criminal acts by a party presently in power. Try them."

Aziz was number 43 on the U.S. most wanted list of Iraqi officials when he gave himself up to U.S. forces in April 2003 just two weeks after Saddam's government fell.

At the previous trial session on Monday, one of the former president's half-brothers gave testimony for Saddam's former intelligence chief. All the eight defendants, including Saddam, were present when Wednesday's session got under way.

They are accused of bloody reprisals, including the killings of 148 men and teenagers, in the Shi'ite town of Dujail after a failed assassination bid on Saddam in 1982.

Elsewhere in Baghdad, gunmen shot dead a Baghdad police general on his way to work on Wednesday, a city official said.

The killing of General Ahmed Dawod, a deputy chief of Baghdad municipality's protection units, appeared to be part of a campaign to assassinate prominent Iraqi officials.

It was another reminder of the security challenge facing Iraq's new government of national unity that took office on Saturday on a pledge to rein in widespread violence plaguing Iraq three years after U.S.-led forces toppled Saddam.

It came a day after three separate bomb attacks killed at least 21 people in the Iraqi capital on Tuesday, including 11 people in a nearby sandwich shop when a bomb hidden in a motorcycle exploded outside a Shi'ite mosque.

TRIBAL CLASHES

New Prime Minister Nuri al-Maliki, who has vowed to use "maximum force against terrorists," said this week his forces could take charge of security in most of Iraq by the end of this year, except for Baghdad and insurgent strongholds in its west.

Security analysts have voiced doubts about the ability of Iraq's fledgling security forces to take over from U.S. and British troops and restore stability in the strife-torn country.

South of Baghdad, clashes between two rival tribes have killed around 16 people, police sources said on Wednesday. Eighteen people were wounded in Tuesday's fighting close to the town of Suwayra, about 40 km (25 miles) south of the capital.

One police source said the violence was linked to land disputes between the two feuding tribes. Another source said it may have been a clash between a Shi'ite and a Sunni tribe.

A Reuters reporter saw 14 bodies that had been taken to Suwayra's hospital.

Police arrested 10 people, a police source said, adding the police and army were now in control of the situation.

While guerrilla and sectarian attacks have killed thousands of people in Iraq since 2003, large-scale fighting between tribes is unusual.

Maliki will be hard-pressed to convince Iraqis that he can make quick progress in the fight against guerillas comprised mostly of Saddam loyalists and al Qaeda militants led by Jordanian militant Abu Musab al-Zarqawi.

He faces a highly sensitive task in choosing interior and defense ministers whose main mission will be to combat Sunni insurgents and check the sectarian violence that erupted after a Shi'ite shrine in the city of Samarra was bombed in February.