The Muslim pilgrims' road to the holy city of Karbala was a highway of bullets and bombs for Shiites on Friday. Drive-by shootings and roadside and bus bombs killed or wounded 19 people, ratcheting up the sectarian tensions gripping Iraq.

Iraqis look at the wreckage of a minibus after a blast, in Shiite district of Sadr city, in Baghdad, Iraq, Friday, March 17, 2006. A plastic bag of explosives covered by cucumbers and tomatoes which was left by a departing passenger, blew up on a minibus in a Shiite district, killing two passengers and wounding four, police reported. [AP]

Security forces, including US armored reinforcements, girded for more bloodshed leading up to Monday's Shiite holiday. And north of Baghdad, in the Sunni Triangle, a two-day-old operation involving 1,500 US and Iraqi troops swept through an area near Samarra in search of insurgents.

It was in Samarra that the insurgent bombing of a Shiite shrine last month ignited days of violence between Shiite and Sunni Muslims. More than 500 people died.

Authorities had feared new attacks as tens of thousands of Shiites, many dressed in black and carrying religious banners, converge on Karbala, 50 miles south of the capital, for Monday's 40th and final day of mourning for Imam Hussein, the Prophet Muhammad's grandson.

The US military announced this week it was dispatching a fresh battalion of the 2nd Brigade, 1st Armored Division, about 700 troops, to Iraq from its base in Kuwait to provide extra security for Shiite holy cities and Baghdad during this period.

Friday's bloodshed in Baghdad began as groups of faithful, many of them parents with children in tow, trekked down city streets headed for the southbound highway to Karbala.

At about 7:30 a.m., a BMW sedan driving alongside pilgrims in the western district of Adil opened fire, killing three young men and wounding two other people, police Lt. Thair Mahmoud said. Police later reported a second shooting, also in western Baghdad, in which men riding in a car fired on pilgrims near Um al-Tuboul Square, wounding three.

Then, about midday, a bomb left in a plastic bag of vegetables exploded on a minibus, killing two passengers and wounding four in a Shiite district of Baghdad, police reported. Later in the day, a roadside bomb went off as a crowd of pilgrims passed in Mahmoudiya, south of Baghdad, wounding five people.

Elsewhere, police in a Shiite area of east Baghdad late Thursday found the bodies of four Sunni men who had been seized from a taxi by masked gunmen the day before in western Baghdad. And police reported that six mortar rounds landed on six houses Friday in a mixed Sunni-Shiite area of Khan Bani Saad, 10 miles north of Baghdad, killing one person and wounding three.

In the western city of Ramadi, US forces again exchanged fire with attackers. The clashes between US troops and insurgents began about 6:30 p.m. Friday around the US base at the provincial government headquarters, according to a doctor at Ramadi hospital, Dheya al-Duleimi. He had no immediate information on casualties.

Iraqi troops killed one attacker in a firefight with insurgents in nearby Fallujah, police Lt. Omer Ahmed reported.

In the big helicopter-borne operation north of Baghdad, only light resistance was reported as some 1,500 troops from the U.S. 101st Airborne Division and Iraq's 4th Division swept through a 100-square-mile area in search of insurgents and weapons.

Lt. Col. Edward Loomis, 101st Airborne Division spokesman, said about 40 suspects were detained, 10 of whom were later released, and six weapons caches were found.

The only casualty reported in the operation was a 101st Airborne soldier shot and killed Thursday while manning an observation post in Samarra.

Two US soldiers from the 101st Airborne Division's Task Force Band of Brothers were killed and another wounded in indirect fire on a base northwest of Tikrit, the U.S. military said Saturday. The attack on Contingency Operating Base Speicher happened Thursday. No names were released.

At least 2,314 members of the U.S. military have died since the beginning of the Iraq war in March 2003, according to an Associated Press count.

"Operation Swarmer," described as the largest air assault operation in three years, was focused on an area of Salahuddin province that was a stronghold of Sunni support for Saddam Hussein's ousted regime.

Speaking by video conference with Pentagon reporters, the US second-in-command here, Lt. Gen. Peter Chiarelli, stressed that the majority of troops in the operation were Iraqi. He said the goal is to have Iraqi security forces in control of 75 percent of Iraq by this summer.

The US command has sought to spotlight development of Iraqi military potential. As Iraqi forces improve, American officials say, US forces in Iraq can be reduced.

Iraqi political leaders, meanwhile, met in another round of talks to break the Sunni-Shiite logjam over the makeup of a new government. They emerged after two hours with no breakthroughs to report.

Minority factions are trying to prevent majority Shiites, the biggest bloc in the new parliament, from dominating the major jobs ¡ª prime minister and defense and interior ministers.

Representatives of the Shiite, Sunni and Kurdish blocs said that on Friday they discussed formation of a National Security Council, a compromise proposal for a joint body to oversee the defense and interior ministries.

More meetings are needed, they said. Tarek al-Hashimi, of the Sunni bloc's Iraqi Accordance Front, said the country faced "a dangerous political dilemma." His Kurdish counterpart, Barham Saleh, said the sectarian crisis runs "much deeper" than the dispute over a Shiite effort to name acting Prime Minister Ibrahim al-Jaafari as the future government chief.

US ambassador Zalmay Khalilzad told The Associated Press on Friday that talks were under way about when he would meet with Iranian officials to discuss the Iraqi political situation. The talks should be held in Baghdad, Khalilzad said.

Iran's Shiite leadership has considerable influence among Iraq's Shiite groups.