“The focus of discussions with the U.S. is Iraq’s security and stability,” said Mohammad Ali Hosseini, the spokesman for the Iranian Foreign Ministry in Tehran, according to IRNA, the official Iranian news agency.

“We are witnessing indiscriminate bombardment of Iraqi residential areas by the U.S. occupying forces,” Mr. Hosseini told reporters at his weekly news conference.

The Americans believe that Iran is supplying weapons and training to Shiite militias in Iraq, but the Iranians have denied that they are interfering here. It remains uncertain whether some of the weapons found in Iraq that appear to be Iranian came directly from Iran or through third parties.

The Iraqis are forming a committee with experts from Interior and Defense Ministries to look into the allegations against Iran.

The State Department did not comment directly on the talks, but a spokesman, Tom Casey, said, “The fact remains, though, that the Iranian government continues, despite their public statements of support for the Iraqi government, to play this negative role, to provide this kind of assistance to militant groups and to militia groups.”

“It’s something we want to see stopped, and it’s something the Iraqis want to see stopped,” Mr. Casey said.

The suspension of talks by Iran is hard to read. It comes on the heels of a disclosure by the American military that among the evidence it has collected of intervention by Iran is documentation of training camps near Tehran run by Lebanese Hezbollah militants. That information was given to an Iraqi delegation to present to Iranian officials last week. But Iranian politics is a game of shadows with so many crosscutting interests that it is hard to say what Iran’s goal may be.

Two things are clear. The talks have not borne much fruit, so suspending them is almost cost free, at least in the short term. The downside is that the talks have been a way for the two countries, which do not have diplomatic relations, to have face-to-face conversations. Several Iraqi politicians said they believed that the Iranian suspension was as much in retaliation for the United States’ criticism of Iran’s nuclear program as it was for Iraq policy.

“Some Iranian officials believe that Iraq is a better location to pressure the Americans over Iran’s diplomatic crisis with them,” Ali al-Dabbagh, an Iraqi government spokesman, told Al Hurra, a satellite channel, on Monday evening.

In addition, Iran loses nothing in Iraq by denouncing strikes on Shiite civilians, especially since it has also said it approves of the Iraqi government’s effort to halt the activities of illegal militias. While those two positions may sound contradictory, they are plausible here. The Iraqi government also says it wants to help civilians and is taking aim only at the militias. The reality is that when forces go after insurgents in urban areas, it is impossible to avoid hitting some innocents as well.

Fellow Shiites are not the only ones disturbed by the fighting; a delegation from Anbar Province, which is almost entirely Sunni, made the long and dangerous journey to Sadr City on Monday to show solidarity with the poor Shiite community. As American helicopter gunships barraged nearby neighborhoods and people ran inside to take shelter, representatives of influential Sunni tribes, including the Janabi and Hebi, stood shoulder to shoulder with distinguished Shiite tribal leaders in Sadr City.

Ali al-Hebi, 40, said, “The people in Sadr city are like our families, and we are worried about them.”

“We must find a solution for them, and we advise them to not repeat our fault in Falluja,” he added. “The loss of any blood is forbidden, according to our religion as Muslims, and if we do not help them, we will be responsible before history for what will happen here.”

The sheiks’ visit was reminiscent of a similar journey by Shiites in 2004, organized by followers of the radical Shiite cleric Moktada al-Sadr. They gathered food and supplies and brought them to Sunnis in Falluja who were besieged by American forces at the time.

American and Iraqi military attacks on Shiite neighborhoods where militias are active included a fierce midnight battle in the Amel neighborhood on Sunday. The American military gave an account of the battle that differed sharply from that of Iraqis in the neighborhood and the Interior Ministry.

According to an Interior Ministry official and neighbors, at least four civilians were killed and as many as 82 wounded. The Americans said that they had returned fire after being fired upon, and that three militiamen had been killed and two civilians wounded and evacuated to a combat support hospital.

According to a neighbor, Haider Jassim, the Americans fired four missiles about midnight, and one hit the house next door, killing a family living in the second-floor apartment. All the other apartments in the building were empty, he said.

“The husband worked in construction, and later I saw the flesh of the mother with her two children — a boy 4 years old and a daughter, who was 1 year old,” he added. “She was burned alive with her mother, who was three or four months’ pregnant with their third child. They were a poor family.”

An American military account of the battle, issued in response to a query, said that small-arms fire was coming from the building, and that after two Hellfire missiles were launched at the building, weapons fire was still coming from it. The Americans launched a third Hellfire missile.

In an unrelated episode of violence, a man in civilian clothes, but carrying an government identification card saying he was a major in the Iraqi Army, approached one of the largest and most fortified checkpoints of the Green Zone, just outside the Assassins’ Gate, a large carved archway. He opened fire on an American soldier at close range, hitting him in the shoulder, according to American and Iraqi witnesses.

“Then the man shot randomly from his Glock pistol; he shot 12 bullets and ran, trying to enter the Green Zone,” said an Iraqi officer, who commands the area’s checkpoints and saw the attack, but asked not to be identified because he was not authorized to speak to the news media. Then an American soldier who was nearby shot the attacker twice.

“When he was wounded, he lay down next to a blast wall and we walked close to him and asked, ‘Why did you do that?’ He said, ‘It was according to an order from my master and my lord,’ and then he vomited a quantity of blood and died,” the Iraqi officer said.

Mohamed Hussein and Qais Mizher contributed reporting.