Also on Monday, the US Embassy confirmed an American is missing in Iraq - presumably one of four Western aid workers who disappeared over the weekend.
The same day, two Sunni Arab politicians were slain in separate attacks - part of an escalation in violence which US and Iraqi officials predicted in advance of the 15 December national parliamentary elections.
The violence occurred as the trial of ousted leader Saddam Hussein for alleged crimes against humanity resumed but was then adjourned until 5 December.
Mortar fire echoed through the centre of the Iraqi capital shortly before the trial began in the heavily guarded Green Zone.
The attack on the pilgrims heading for Karbala took place in the southwestern Baghdad neighbourhood of al-Dura, one of the most dangerous parts of the city.
Aljazeera reported that armed men also attacked a local TV team on their way to the pilgramage, killing two of the team members.
The pilgrims were identified as Saifulddin Makai, 39, and Husain Mohammedali, 50, both business people from the London area, friends and associates said. The three wounded pilgrims were also British Muslims.
The British Muslims were on a
pilgrimage to Shia holy sites
In Baghdad, deputy Interior Minister Hussein Kamal said authorities had no leads into the weekend disappearance of four Western humanitarian workers. No group has claimed responsibility and details of the apparent kidnapping were unclear.
On Sunday, a Canadian official, Dan McTeague, said the four included two Canadians.
Britain has said one of its citizens, retired professor and longtime peace activist Norman Kember, had gone missing in Iraq, but refused to say whether he was among the four.
US Embassy spokeswoman Elizabeth Colton said only an American had been reported missing and that the person's name and organisation were being withheld.
The four were believed to be affiliated with Christian Peacemakers, one of the few Western humanitarian organisations still operating in Iraq. The organisation's website states that, among other things, the group seeks to expose and document abuses of Iraqis by US and multinational forces.
Four US soldiers were wounded
in an attack on their Humvee
In Barcelona, Spain, British Foreign Secretary Jack Straw said he had contacted Iraqi Foreign Minister Hohshyar Zebari about Kember's abduction, and that Zebari "pledged every assistance from the Iraqi government".
"It is a very difficult and very worrying situation for the family, obviously for Mr Kember himself, and we remain in touch with the family," Straw said.
Britain's Foreign Office said on Sunday that it was launching an "urgent investigation" and would be in touch "with the other countries involved, the Americans and the Canadians".
Although violence is expected to escalate before next month's election, US and Iraqi authorities hope the ballot will over time help calm the anti-government campaign - if the new parliament includes a large number of representatives from the disaffected Sunni Arab community, the backbone of the uprising.
Call to vote
Many Sunni Muslims boycotted the January election, enabling Shia and Kurds to win an overwhelming majority and sharpening communal tensions. This time, however, many leading Sunni groups are fielding candidates, and Sunni clerics are urging their followers to go to the polls.
All that could be threatened by pre-election violence, however.
In the latest incidents, armed men killed a senior official of Iraq's largest Sunni Muslim party and his two bodyguards as they drove on Monday from Falluja to Baghdad, the Iraqi Islamic Party told Aljazeera.
"It is a very difficult and very worrying situation for the family, obviously for Mr Kember himself, and we remain in touch with the family"
British Foreign Secretary
Ayad al-Izzi, a member of the party's political bureau, was a candidate in the parliamentary election.
Iraqi police force in Baiji said that Shaikh Khalaf Ali al-Sulayman, one of the leaders of the Shammar tribe, was killed when US soldiers opened fire after their patrol was hit by an explosive device.
The US army announced that four US soldiers were wounded when a bomber driving a booby-trapped car attacked them in Baquba.
Three US soldiers were also injured when an explosive device blew up targeted a US personnel carrier east of Baghdad.
Meanwhile, Aljazeera reported that a public affairs officer for the Sunni-led Council for National Dialogue was shot and killed on Monday in southern Baghdad, police said.
No group claimed responsibility for either attack. Al-Qaida in Iraq, led by Jordanian-born Abu Musab al-Zarqawi, has warned Sunni Muslims against participating in the election, although political rivals could also be responsible.
Several Iraqi tribal chiefs have
been killed in ongoing violence
Iraq was rocked by a wave of foreigner kidnappings and beheadings in 2004 and early 2005 including al-Qaida in Iraq seizing more than 225 people, killing at least 38 of them -including three Americans.
The victims included aid workers, journalists and contractors, seized in an attempt to drive foreigners out of the country or to win large ransoms.
Since May, abductions have dropped off considerably, mainly because many Western groups left Iraq and security precautions for those remaining have been tightened, with foreigners staying in barricaded compounds and moving only in heavily guarded convoys.
The last American to be kidnapped was Jeffrey Ake, a contract worker from LaPorte, Indiana, who was abducted on 11 April. He was seen in a video aired days afterwards, held with a gun to his head, but there has been no word on his fate since.
Talks with Iran?
The US ambassador to Iraq has been given permission to meet officials from Iran, a country with no diplomatic relations with the US, the State Department said on Monday.
"It's a very narrow mandate that he has," spokesman Sean McCormack said of Ambassador Zalmay Khalilzad, "and it deals specifically with issues related to Iraq."
The US has not had regular diplomatic relations with Iran since the Islamic revolution in 1979.
"If there is a particular issue to discuss, to try to resolve, ... I'm sure that there will be a meeting"
State Department spokesman
McCormack, however, played down the development, which was reported first in Newsweek magazine, saying there had been previous cases where US officials had engaged Iran, including talks on rebuilding Afghanistan.
He said he lacked specifics of what might be included in future talks, or whether any meetings had already occurred, but he said possible topics could involve questions about the Iraq-Iran border.
"If there is a particular issue to discuss, to try to resolve ... I'm sure that there will be a meeting," McCormack said of Khalilzad's mandate.