South of Baghdad, a suicide tow truck driver killed at least 24 people with a huge bomb in the Shiite city of Hillah. Police and morgue officials said a total of 58 people, including the Hillah victims, were killed or found dead nationwide.

Speaking to reporters after a second session in two months with the Iranian envoy, Ambassador Ryan Crocker called the seven-hour meeting "full and frank," diplomatic language for difficult.

The Bush administration does not appear to expect much if anything from the talks but seems willing to go forward with them because the high-powered and bipartisan Iraq Study Group, in a report late last year, recommended contacts with both Iran and Syria in a bid to end or ameliorate outside influences in Iraq as part of a plan to end the conflict.

For its part, Iran appears to be enjoying the spectacle and prestige of negotiating with world's only superpower after more than a quarter-century freeze in open diplomatic contact.

"We discussed ways forward, and one of the issues we discussed was the formation of a security subcommittee that would address at an expert or technical level some issues relating to security, be that support for violent militias, al-Qaida or border security," Crocker said.

But he warned progress was impossible until Iran matches its behavior on the ground with its declarations backing an independent and stable Iraq.

"The fact is, as we made very clear in today's talks, that over the roughly two months since our last meeting we've actually seen militia-related activity that could be attributed to Iranian support go up and not down," Crocker said, citing testimony from detainees and confiscated weapons and ammunition as evidence.

"We made it clear to the Iranians that we know what they're doing (and) it's up to them to decide what they want to do about it," he said.

In a later conference call with reporters in Washington, Crocker said portions of the long exchange were heated.

"I would not describe this as a shouting match throughout, but we were real clear on what our problems with their behavior was, and I just didn't hesitate to let them know," Crocker said.

Crocker said he expected the session to be testy, given the extensive list of U.S. complaints and the overall difficulty of the relationship.

"We've got a lot of problems with the Iranians, and face to face we're not going to pull any punches," Crocker said.

In a separate news conference after the talks, Iranian Ambassador Hassan Kazemi Qomi countered that Tehran was helping Iraq deal with the security situation but Iraqis were "victimized by terror and the presence of foreign forces" on their territory.

He said his delegation also demanded the release of five Iranians detained by U.S. forces in Iraq. The United States claims the five were linked to Iran's elite Quds Force, which Washington accuses of arming and training Iraqi militants. Iran says the five are diplomats who were legally in Iraq.

"There are also Iranian citizens who have been detained on legally entering Iraq," Qomi said. "We demanded their release too. We discussed the creation of a mechanism to implement what we achieved in the first round of talks. They (the Americans) acknowledged making mistakes and this is a step forward in itself and it's now up to the Americans to rectify their mistakes."

He did not say what those mistakes were.

Qomi told The Associated Press that 20 to 30 other Iranian citizens were in U.S. custody.

The detention of four Iranian-Americans in Iran has deepened tensions between Washington and Tehran, whose relations were already strained over Iran's nuclear program and its support for radical militant groups like Lebanon's Hezbollah and the Palestinian Hamas and by U.S. military maneuvers in the Persian Gulf.

Iraqi Foreign Minister Hoshyar Zebari, who chaired the meeting, said experts would meet as early as Wednesday to work out the structure and mechanism of the committee.

"We hope that the next round of talks will be on a higher level if progress is made," he said at the news conference.

The meeting, held in Prime Minister Nouri al-Maliki's offices, was a rare face-to-face discussion for U.S. and Iranian representatives, whose nations have been estranged for nearly three decades.

The session was only the second of its type £­ a formal, direct meeting between high-level representatives of each nation that was revealed publicly in advance. Other publicly known contacts have been more casually arranged on the sidelines of larger international gatherings, or by happenstance. That was the case when then-Secretary of State Colin Powell dined beside his Iranian counterpart at a 2004 diplomatic dinner.

U.S. and Iranian envoys also met several times to discuss cooperation in securing Afghanistan after the fall of the Taliban regime in 2001.

Al-Maliki opened Tuesday's meeting, issuing an impassioned appeal for help from the two nations to stabilize Iraq and warning that militants from al-Qaida and other terror groups in Iraq were now fleeing and finding refuge elsewhere.

"We are hoping that you support stability in Iraq, an Iraq that doesn't interfere in the affairs of others nor wants anyone to meddle in its own affairs," he said, according to excerpts of al-Maliki's remarks released by his office.

"It's Iraq's right to call on everyone to stand beside it to counter the scourge of terror and extremism," he said. "The world ... must stand together and face this dangerous phenomenon and its evils, which have gone beyond the borders of Iraq after terror and al-Qaida groups received strong blows and are now running away from the fight and moving to other nations."

The Hillah bomber struck at 9 a.m., according to provincial police, who said the driver of the tow truck detonated his payload in the middle of the Bab al-Mashhad district. Iraqi troops cordoned off the area while fire engines and ambulances rushed in.

Most of the 24 killed and 69 wounded in the blast suffered serious burns, said Ayad Abdul-Zahra of Hillah hospital.

Eassam Rashid, 32, was selling vegetables at his stall when the blast sent shrapnel flying over his head.

"I heard a tremendous explosion followed by a fireball," he said. "Then nearby cars were set ablaze one by one, and I saw four or five people struggling to get out of their burning cars."

Hillah, about 60 miles south of Baghdad, has been the site of some of the deadliest bombings, including a double suicide attack on March 6 that killed 120 people.