Iraq's prime minister called on Syria (search) on Thursday to block the infiltration of foreign fighters trying to start a civil war. More than a dozen Iraqis, including an Oil Ministry engineer, and four U.S. soldiers were reported killed in the ongoing daily bloodshed.

In the day's deadliest incident, a Sunni (search) lawmaker said 10 of his private guards were killed during a chaotic battle with insurgents and Apache helicopter-backed U.S. forces, whom he accused of killing several of his aides. The U.S. military said it was investigating the incident, which it called a terrorist attack on al-Jarba's home.

In a message aimed at Syria, Prime Minister Ibrahim al-Jaafari (search) said non-Iraqis in neighboring countries filtering across the border were responsible for carrying out "sabotage activities."

His government will do its best "to preserve relations between us and our neighbors, but there are red lines — which are that they don't interfere in our internal affairs, or allow people to cross" into Iraq, he said after a meeting with visiting U.S. Deputy Secretary of State Robert Zoellick.

Earlier this week, al-Jaafari pledged to use "an iron fist" to prevent an outbreak of sectarian violence which he has accused supporters of Iraq's top Al Qaeda terrorist, Abu Musab al-Zarqawi (search), of trying to foment.

Zoellick said he and al-Jaafari discussed the issue of insurgents crossing into Iraq "and he was quite strong in his statements about the need for Iraq's neighbors, and particularly Syria, you know, not to undermine stability here."

Secretary of State Condoleezza Rice made it "quite clear that we, and others, are watching how Syria behaves itself," Zoellick told reporters.

"Part of these attacks stem from the successes of the new government. The insurgents wanted to stop the elections and failed. The insurgents wanted to stop the formation of a new Iraqi government and they failed, so now they are trying to split the society," Zoellick said.

Another insurgent goal, he said, is to "disrupt the reconstruction so that people lose confidence in the future."

The Syrian government has not commented on allegations that foreign fighters are both slipping into Iraq across its shared border and holding clandestine planning meetings in its country. But in February, Syria captured and handed over Saddam Hussein's half brother in what Iraq called a gesture of goodwill.

A U.S. official said Wednesday that Syria was the site of a key meeting last month in which al-Zarqawi lieutenants were ordered to carry out more attacks in Iraq. More than 520 people have been killed since the country's new Shiite-dominated government was announced April 28.

On Friday al-Jaafari is slated to start a two-day visit to neighboring Turkey, his first foreign trip since becoming prime minister. The insurgency is expected to top the agenda.

The campaign of violence has made the threat of civil war increasingly tangible.

A number of clerics, both Shiite and Sunni, have been killed, and on Thursday three of Iraq's most influential Sunni Muslim organizations called on mosques to shut for three days following Saturday's dawn prayer to protest alleged Shiite violence against them.

In the attack on National Assembly member Fawaz al-Jarba, the Sunni lawmaker said a group of insurgents started shooting at his house in Mosul, 225 miles northwest of Baghdad, around midday.

His personal security men returned fire, sparking a clash that raised the attention of U.S. soldiers in the area, al-Jarba told The Associated Press.

"The firing was coming from all directions and the Apaches were bombing," said al-Jarba. "The terrorists were firing at me. Why did the Americans start doing this too?"

Al-Jarba, a recent candidate for parliament speaker, said one Apache helicopter flying over his home fired at it, leaving a large hole in his ceiling. Eight of his bodyguards were killed during the initial battle and two died from their wounds later Thursday in hospital, he said. The U.S. military said three terrorists were also wounded in the incident.

One U.S. soldier was killed when his convoy struck a roadside bomb and another two died from wounds suffered when insurgents fired on their convoy Thursday, the U.S. military said. Both incidents occurred in Baghdad. Another American soldier was killed and five wounded late Wednesday in a rocket attack on Camp Ar Ramadi, 70 miles west of Baghdad. The soldiers' names were being held pending notification of family.

Three American soldiers were wounded when a roadside bomb blast struck their patrol in Dujayl, 35 miles north of Baghdad, said military spokesman Sgt. David Rhodes.

At least 1,627 members of the U.S. military have died since the Iraq war started in March 2003, according to an Associated Press count.

An Oil Ministry employee also was gunned down in front of his home. The gunmen shot and killed 31-year-old Ali Hamid Alwan al-Dulaimy and then sped off. Late Wednesday, an official from the Youth and Sport Ministry was killed in another drive-by shooting. Insurgents have stepped up assassinations of Iraqi officials in recent weeks.

Also Thursday:

—In Baghdad, a homicide bomber drove his car into an Iraqi army checkpoint, killing one soldier and injuring another eight, Iraqi army Capt. Firas Aied said.

— Two Iraqi police officers were killed by a roadside bomb in Baqouba, 35 miles northeast of Baghdad.

— In Samarra, north of Baghdad, gunmen assassinated policeman Omar Majeed Shakir al-Dosh and his father, officials said.

— In Ramadi, west of Baghdad, the body of a man who had been kidnapped early Thursday was found dumped on a street with a note warning Iraqis not to collaborate with Americans.

On Wednesday, gunmen assassinated Shiite cleric Mohammed Tahir al-Allaq, representative of Grand Ayatollah Ali al-Sistani in the Jurf al-Nadaf area near Madain, about 14 miles southeast of Baghdad, said police Lt. Col. Shakir Wadi Hamoud said.