BAGHDAD — American military officials yesterday accused a Shiite terrorist group of carrying out a truck bombing in northwestern Baghdad on Tuesday evening that killed at least 65 people, the deadliest attack in the capital since March.

The accusation was startling because the bombing in the Hurriyah neighborhood had the hallmarks of earlier large-scale attacks in predominantly Shiite areas that had been attributed to Sunni terrorist groups such as Al Qaeda in Iraq.

An American military spokesman said intelligence reports indicate that the leader of a Shiite “special group,” Haydar Mehdi Khadum al-Fawadi, planned the bombing in an effort to fuel animosity toward Sunnis in the largely Shiite district. The American military uses the term special groups to describe what it says are smaller Iranian-backed militias.

“It’s very troubling,” an American military spokesman in Baghdad, Major General Kevin Bergner, said. “It indicates a desire to incite sectarian conflict, sectarian backlash.”

The Iraqi government has conducted military operations this year in Basra, Mosul, and Sadr City, a Shiite district in eastern Baghdad, to crack down on militias and insurgents.

The government’s next operation, in the southeastern province of Maysan, is expected to start this week. American and Iraqi troops have arrived in the provincial capital, Amarah, in recent days. The city, which is near the Iranian border, is a gateway for weapons and Iraqi fighters alleged to have been trained in Iran.

Unlike the operation in Basra in March, which led to sustained clashes between Iraqi soldiers and militiamen that quickly spread to Baghdad, the Iraqi government has given Amarah residents plenty of notice and urged them to disarm.

Local followers of the anti-American cleric, Moqtada al-Sadr, said they would not provoke the security forces as long as the operation does not entail abuses.

Adnan al-Silawi, who heads the Sadr office in Amarah, said militiamen have been instructed not to carry weapons on the street and encouraged to cooperate with the army.

“It should be like a state going into one of its provinces and not like an army conquering a city,” he said.

[President Bush would win $162 billion in long-overdue funding to carry out military operations in Iraq and Afghanistan into next year under a bipartisan agreement sealed on Capitol Hill yesterday, the Associated Press reported. The agreement reached between House Democrats and Republicans and the White House — if passed into law as expected — would finally put to rest Mr. Bush’s long-standing battles with congressional Democrats over war funding.]