Suspected Sunni gunmen killed 23 policemen Sunday, including 17 in one attack in the predominantly Shiite southern city of Basra, signaling the possible start an all-out insurgent campaign against Iraq's predominantly Shiite security forces.

Political tension deepened in Baghdad when Vice President Tariq al-Hashemi, the country's ranking Sunni politician, threatened to resign if Prime Minister Nouri al-Maliki did not act fast to eradicate two feared militia groups

Al-Maliki depends heavily on the backing of a pair of Shiite political organizations and has resisted concerted American pressure to eradicate their private armies — the Mahdi Army of radical anti-American cleric Muqtada al-Sadr and the Badr Brigade, the military wing of Iraq's most powerful Shiite political bloc, the Supreme Council for the Islamic Revolution in Iraq, or SCIRI.

The gunmen, especially those of the Mahdi Army, are deeply involved in the sectarian killings that have brutalized Iraqis in Baghdad and central Iraq for months.

The sharp uptick in violence against the mainly Shiite security apparatus coincided with U.S. efforts to bring the country's Sunni insurgents into a reconciliation process and an embarrassing public squabble with al-Maliki over a proposed timeline to crush the militias and move forward with political measures to benefit the disaffected Sunni minority.

Killings Sunday — at least 33 in all, not counting the discovery of the bodies and decapitated heads of 24 other victims — ended five days of relative post-Ramadan calm. Sectarian violence flared and has burned ever hotter since Sunni bombers destroyed a major Shiite shrine in Samarra, north of the capital, Feb. 22.

Until then, the Sunni insurgency held a virtual monopoly on violence, both in attacks against American forces and massive bomb attacks against Shiite civilians. Since Shiite militias and roaming death squads joined the fight, thousands have been tortured and killed on both sides of the sectarian divide, taking Baghdad and other mixed Shiite-Sunni regions in the center of the country to the brink of civil war.

In the Basra kidnap-murders, gunmen dragged 15 policemen and two translators — instructors at the Basra police academy — off a bus at the edge of the city Sunday afternoon. Their bodies were found dumped throughout the city beginning about four hours later.

Fourteen of the victims were Shiite, three were Sunni. Shiites are about 80 percent of the population in Basra, Iraq's second largest city.

Political tremors compounded the destabilizing violence when the top aide to al-Hashemi, the vice president and highest-ranking Sunni politician, said his boss had threatened to resign.

"Yes, the vice president is threatening to quit. Our position dictates that we send a message to the government. We cannot live with this situation indefinitely," said Mohammed Shaker, the key aide.

Al-Hashemi's threat, whether genuine or a political pressure tactic, would rock the already shaky al-Maliki government.

In an apparent bid to shore up support among his Shiite base, the prime minister issued an angry series of statements last week, denouncing U.S. plans for a timeline to measure progress as an infringement of Iraqi sovereignty.

On Saturday, a top aide to al-Maliki said, after the prime minister held a video conference with U.S. President George W. Bush, that the Iraqi leader also was using American voter dissatisfaction with the war as a lever to strengthen his position with the United States.

The public spat arose after U.S. Ambassador Zalmay Khalilzad outlined adjustments in U.S. policy that called for al-Maliki set timelines for progress on a series of tasks, including disbanding militias.

"Success in Iraq is possible and can be achieved on a realistic timetable. Iraqi leaders must step up to achieve key political and security milestones on which they have agreed," Khalilzad said in a bid to put a new face on the increasingly unpopular war with just days remaining before midterm elections.

Al-Maliki denied he had agreed to anything and declared at one point:

"I am a friend of the United States, but I am not America's man in Iraq."

Hassan al-Suneid, a key aide, also said that al-Maliki had complained to Bush about what the prime minister saw as imperious treatment from Khalilzad.

In an interview with CNN on Sunday, Khalilzad said the squabble was a misunderstanding.

"That was a problem in how what I said was interpreted or translated to him and how it was played by some of the media here. What he understood as it was explained to him was that I had determined what issues and by when the Iraqis had to decide," the ambassador explained.

Iraqi court officials, meanwhile, said a verdict in Saddam Hussein's first trial would definitely be handed down on Sunday, two days before the American election.

Saddam faces death by hanging in that case brought in connection with the killing of 148 Shiite villagers in Dujail after a 1982 assassination attempt against him. Saddam is being tried with seven co-defendants. If found guilty, an appeal to a higher court is automatic.

The ousted leader's lawyer warned that violence would spread in Iraq and could wreck havoc throughout the Middle East if Saddam is sentenced to hang.

Khalil al-Dulaimi also said he would attend Saddam's resumed hearings Monday on separate charges of genocide against the Kurds, breaking a monthlong boycott of the trial.

The lawyer told The Associated Press in Jordan that he warned of a civil war in a recent letter to Bush.

"I warned him against the death penalty and against any other decision that would inflame a civil war in Iraq and send fire throughout the region," al-Dulaimi he said. He did not say precisely when or how he had transmitted the letter to Bush.