Baghdad, Iraq ? Militant leader Abu Musab al-Zarqawi met with heads of Iraqi insurgent groups in Syria a month ago and called for a shift in strategy against Iraqi and U.S. forces by increasing suicide car bombings, a senior U.S. military official said Wednesday.

The official said that shortly after the meeting, held just inside the Syrian border, insurgents unleashed dozens of car bombs throughout Iraq as part of a wave of violence that in recent weeks has killed more than 450 people. There have been 21 car bombings so far in May in Baghdad. That compares with about 25 such attacks in all of 2004. Since Feb. 27, according to the U.S. military, 126 car bombs have either exploded or been diffused in the capital.

“Zarqawi was not happy with how the insurgency was going,” said the official, requesting anonymity because of the sensitive security nature of the information.

U.S. officials said al-Zarqawi’s emphasis on car bombings suggested a frustration among extremists attempting to sustain a war against 140,000 U.S. soldiers and a growing Iraqi army.

The senior official added that various intelligence sources reported that al-Zarqawi and various guerrilla leaders had attended at least five meetings in Syria and western Iraq during the past year. The official said there was no indication the Syrian government was aware of the gatherings.

The official’s comments could not be independently verified, and it was not known how U.S. officials gleaned information about a clandestine insurgent meeting.

A message posted on an Islamic Web site Wednesday and attributed to al-Zarqawi sought to justify the car bombings that surged after the new Iraqi government named its Cabinet April 28.

“The killing of infidels by any method including martyrdom (suicide) operations has been sanctified by many Islamic scholars even if it means killing innocent Muslims. This legality has been agreed upon … so as not to disrupt jihad,” the message said.

When asked whether the insurgency was becoming more formidable, the senior military official said: “I don’t think they’re gaining strength. I think they’re changing their techniques and tactics.”

Increased reliance on car bombs by insurgents complicates the new Iraqi government’s handling of the war while it struggles with a surge in assassinations of public officials and sectarian violence between Shiites and Sunni Arabs. Gunmen firing from a sedan Wednesday killed a senior member of the Iraqi Interior Ministry, Brig. Gen. Ibrahim Khamas, as he drove in southeastern Baghdad.

The U.S. military official said it was unclear to U.S. forces whether the tit-for-tat slayings between religious groups were foreshadowing a civil war or rather represented a re-emergence of clan animosities that were contained by Saddam Hussein’s police state.

“It’s hard for us to see right now,” the official said.