Baghdad, Iraq ? Gunmen ambushed a bus Monday carrying British Muslims to Shiite shrines, killing two Britons and wounding three. The U.S. Embassy confirmed an American is missing in Iraq – presumably one of four aid workers who disappeared over the weekend.

Also Monday, two Sunni Arab politicians were slain in separate attacks – part of an escalation of violence that U.S. and Iraqi officials predicted in advance of Dec. 15 parliamentary elections.

The attack on the pilgrims took place in the southwestern Baghdad neighborhood of Dora, one of the most dangerous parts of the capital. The victims were en route to Shiite holy cities of Karbala, Najaf and Kufa, according to friends in Britain.

Those killed were identified as Saifuddin Makai, 39, and Husain Mohammedali, 50, both businessmen from the London area, friends and associates said. The three wounded pilgrims were also British Muslims.

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 Norman Kember, had vanished in Iraq but refused to say whether he was among the four.

Kember is a longtime peace activist who once fretted publicly that he was taking the easy way out by protesting in safety at home while British soldiers risked their lives in Iraq.

A U.S. Bradley fighting vehicle burns Monday in Baghdad, Iraq, after a roadside bomb detonated next to a passing U.S. Army convoy.

“He’d been involved in peace and nonviolence programs for most of his life, constantly challenging wars and violence and committed to supporting conflict resolution programs,” said Pat Gaffney, an adviser to the Christian Peace Education Fund, where Kember is a trustee.

U.S. Embassy spokeswoman Elizabeth Colton said only that an American had been reported missing and that the person’s name and organization were being withheld.

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.”

Although violence is expected to rise before the election, U.S. and Iraqi authorities hope over time the ballot will help calm the insurgency – if the new parliament includes a large number of representatives from the disaffected Sunni Arab community, the backbone of the rebellion.

Many Sunnis boycotted the January election, enabling Shiites 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.

In the latest incidents, gunmen killed a senior official of Iraq’s largest Sunni party and his two bodyguards as they drove Monday from Fallujah to Baghdad, the Iraqi Islamic Party said. Ayad al-Izzi, a member of the party’s political bureau, was a candidate in the parliamentary election.

Ghalib al-Sideri, a public relations director for the Sunni-led Council for National Dialogue, was shot and killed 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 Sunnis against participating in the election, although political rivals could also be responsible.