BAGHDAD (AP) - Suspected Sunni Arab gunmen killed 23 policemen yesterday, including 17 in one attack in the predominantly Shiite southern city of Basra, signaling the possible start of an intensified insurgent campaign against Iraq's predominantly Shiite Muslim security forces.

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

Al-Maliki, a Shiite, depends heavily on the backing of the two Shiite political parties that run the militias and has resisted American pressure to eradicate the private armies - the Mahdi Army of radical cleric Muqtada al-Sadr and the Badr Brigade, the military wing of Iraq's biggest Shiite political bloc, the Supreme Council for the Islamic Revolution in Iraq.

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

Police have been targeted throughout the insurgency in an effort to destabilize the U.S.-supported government, but the number of attacks Sunday was a sharp step-up in attacks aimed at security services.

The surge in violence also comes amid U.S. efforts to bring Sunni insurgents into a reconciliation process and a public squabble with al-Maliki over a timeline for crushing the Shiite militias and moving forward with measures to soothe the disaffected Sunni minority.

Thirty-three people in all were slain across Iraq yesterday in a second day of rising violence that ended a five-day lull in attacks after the end Muslims' holy month of Ramadan.

Officials also found 24 bodies, some of them decapitated, the latest victims of sectarian reprisal killings that have soared since Sunni bombers destroyed a Shiite shrine in Samarra, north of the capital, Feb. 22.