A recent wave of killings and assassinations in Iraq threatens a tenuous political truce between Arab Sunni Muslims and Shiite Muslims, the head of the country's most influential group of Sunni clerics said Saturday, 12 days before national parliamentary elections.

Abdul Salam Kubaisi, a senior official of the Sunni Association of Muslim Scholars, said the group would "reconsider the decisions" it reached with rival Shiites and Kurds at a national reconciliation conference last month in Cairo. The leaders had agreed that violence in Iraq should stop, U.S. forces should gradually withdraw and some detainees should be freed.

The agreement was hailed by the United States and other governments as an important step toward preventing the country from splitting into warring factions. But Kubaisi said the pledge to curb the violence had not been kept. He blamed the Shiite-led government's security forces and U.S. troops for the continuing attacks.

"What is happening on the ground differs completely from what was promised," he said at a news conference.

The Sunnis and Shiites have accused armed groups belonging to each other's sects of carrying out assassinations, bombings and abductions in advance of the Dec. 15 elections.

The violence continued Saturday as 17 Iraqi soldiers were killed in an ambush near Baqubah, 35 miles northeast of Baghdad. A bomb exploded near the army patrol, and gunmen then opened fire, according to Ali Khayam, an Iraqi army spokesman.

An Iraqi policeman was killed in Samarra, about 65 miles north of Baghdad, during an hour-long gunfight. Another officer was killed in the northern city of Kirkuk.

U.S. military authorities also announced Saturday that three American soldiers had been killed in a vehicle accident in Balad, about 50 miles north of Baghdad, and that a U.S. Marine had died of wounds from a rocket attack Thursday in Ramadi. U.S. forces have suffered 22 fatalities in the past five days.

Kidnappings have also resumed after a lull. In Baghdad, U.S. forces carrying out a routine car search found two civilians bound and gagged in the trunk of the vehicle Thursday, a military spokesman said Saturday. Two men in the car carrying fake Iraqi police badges were arrested, the spokesman said.

Al-Jazeera, the Arabic television satellite network, broadcast a second video showing four Western peace activists, including Tom Fox, 54, of Clear Brook, Va., who were abducted a week ago. Al-Jazeera did not broadcast the audio from the tape but said it included a threat to kill the four hostages if all detainees in U.S. and Iraqi prisons were not released by Thursday. The network broadcast a previous video of the captives Tuesday.

Violence in Iraq is threatening to tear the country into pieces controlled by sectarian rivals, heightening the risk of a civil war that could draw in neighboring countries. Many suspect that Sunni Arabs, who dominated the government until Saddam Hussein was overthrown as president, are encouraging the insurgency.

The head of the largest Shiite political party reiterated that prospect Saturday in a warning to Sunni Arab rivals. "We will not allow the Saddamists and Baathists to return to the government and its institutions," said Abdul Aziz Hakim, head of the Supreme Council for the Islamic Revolution in Iraq. He added that the party would "work to form regions," a formula likely to give the majority Shiites an oil-rich base in southern Iraq.

The country's most revered Shiite cleric, Ayatollah Ali Sistani, on Saturday urged his followers to participate in the elections, though he did not endorse any political party.

Some Sunnis say the government's security apparatus, including the army and police, has been infiltrated by Shiite militias that are carrying out attacks in league with the United States.

Kubaisi said the Association of Muslim Scholars might stop cooperating on political matters because of the "continuing breaches of human rights violations by the Iraqi forces and American occupation forces." He brandished pictures of a dead man and his 1-year-old son who, he said, were killed by Iraqi forces, as well as pictures of three farmers he said were killed by U.S. forces west of Baghdad. These were "crimes against humanity committed by the Americans against helpless civilians," he said.

Kubaisi's talk of withdrawal "is a powerful threat," said Khalaf Elayan, head of the National Dialogue Council, a Sunni political party aligned with the association. "If they decide to withdraw from the political process, we will withdraw as well. We are all gathered under one umbrella."

If the Sunni Arab parties boycott the elections as they did the January election for the current government, "those who were marginalized before will be marginalized again," he said.

Naseer Ani, head of one of the largest Sunni political parties in the bloc, said a decision had not been made on whether to participate in the elections. But he said the parties were unhappy that the Cairo conference had produced few results.

"The Cairo conference agreed to stop the violations and raids against Sunnis," he said. "But that didn't happen."

Fresh evidence of the tactics used by both sides was provided in the form of a compact disc that was given to a reporter in Tikrit by a Sunni mosque official. The disc shows a masked man speaking for the Mujaheddin Army, a Sunni-led insurgency group, and two women, identified as Najal and Suad.

The two women, who worked with Iraqi and U.S. forces, are shown nervously "confessing" to having been used sexually. They are blindfolded, and then they are shot. The bodies were found Nov. 21 in the desert near a U.S. base.

A spokesman for U.S forces said the military has not seen the CD and had no comment.

Correspondent Jonathan Finer and special correspondents Omar Fekeiki in Baghdad, Salih Saif Aldin in Tikrit and Hassan Shammari in Adhaim contributed to this report.