BAGHDAD - Iraqi Prime Minister Nuri al-Maliki's unity government plunged deeper into crisis on Monday when four secularist ministers withdrew from cabinet meetings, less than a week after the main Sunni Arab bloc quit.

A total of 17 ministers, nearly half of Maliki's cabinet, have now quit or are boycotting meetings at a time when he is under growing pressure from frustrated U.S. officials to make demonstrable progress in reconciling Iraq's warring sects.

"We are still in the government but we are boycotting cabinet meetings," said Human Rights Minister Wijdan Michael, one of the four ministers from the secularist party of former interim Prime Minister Iyad Allawi.

Earlier on Monday, a truck bomber in a crowded residential area killed at least 33 people in their homes.

The cabinet boycott means Maliki sets off on Tuesday for visits to Turkey and Iran this week, with signs he is losing control of his government back home.

In Baghdad, U.S. diplomats met held the first meeting of a new security committee with officials from Iran.

Establishing the security sub-committee has been the main achievement so far of new face-to-face contacts between Washington and Tehran, who have had no diplomatic ties for almost 30 years but were driven to the negotiating table earlier this year by the threat of all-out civil war in Iraq.

"It is an established channel of communication and we will see in the future as to whether or not it is a useful channel of communication," U.S. State Department spokesman Sean McCormack told reporters in Washington. A U.S. embassy official called the first meeting, which last four hours, "frank and serious".

After it ended, ambassador Ryan Crocker met his Iranian counterpart Hassan Kazemi-Qomi for two hours of further talks in the office of Iraqi security adviser Mowaffak al-Rubaie.