Suicide bombings have become the Iraqi insurgency's weapon of choice, with a staggering 90 attacks accounting for most of last month's 750 deaths at the militants' hands, according to tallies by the U.S. military and news agencies.

Suicide attacks outpaced car bombings almost 2-to-1 in May, according to those tallies. In April, there were 69 suicide attacks -- more than in the entire year preceding the June 28, 2004, hand-over of sovereignty.

A suicide car bomber attacked the main checkpoint to Baghdad's international airport yesterday, wounding 15 Iraqis, the U.S. military said. The car bomb exploded by a security checkpoint where Iraqi workers were waiting in long lines in their vehicles to enter the sprawling airport grounds.

Hussein Muhsen, an aircraft engineer with Iraqi Airways, described a loud blast followed by a mountain of dust rising in the air. Blood and human limbs splattered down on the hoods and roofs of the cars, Muhsen said.

U.S. forces said insurgents opened fire after the attack. Witnesses said U.S. troops also fired. The airport road has been one of the most frequent scenes of insurgent attacks.

Iraq has experienced daily bombings for months, and yesterday was one of the first days in weeks in which no fatalities other than the bomber were immediately reported from such attacks.

The frequency of Iraq's suicide bombings is unprecedented, exceeding the practice through years of the Palestinian uprising against Israel and other militant insurgencies, such as the Chechen rebellion in Russia. Baghdad alone saw five suicide bombings in a six-hour span Sunday.

With U.S.-led forces now better protected with concrete blast walls and concentric rings of concertina wire and sandbags, the militants have taken to targeting Iraqi police and civilians in their bid to convince Iraqis that their new leaders can't protect them.

Increasingly, Iraqis are believed to be carrying out at least some of the suicide attacks.

U.S. officials and Iraqi analysts say the insurgents' resources are increasing on several fronts: money to buy cars and explosives, expertise in wiring car and human bombs, and intelligence leaks that help the insurgents target U.S. and Iraqi forces.

Suicide attacks are on the rise because the explosive devices "are simple to construct and easy to operate, thus making suicide bombers difficult to detect," said Navy Cmdr. Fred Gaghan, in charge of the Combined Explosive Exploitation Cell in Iraq, which studies bomb scenes for clues to insurgent tactics.

"At this time, there is nothing to indicate that the availability of volunteers is on the decline," he said, noting the media coverage and videos of suicide bombings posted on the Internet that fuel extremist recruitment.

Saad Obeidi, a retired Iraqi major general and security expert, suggested that President Bush had invited Islamic extremists to bring to Iraq their fight against the United States. "One aim of the U.S. military, once it invaded Iraq, was to lure all insurgents and terrorists from all over the world to confront them here," he said.

The first suicide bombings of the insurgency were attributed to foreign infiltrators -- mostly Palestinians, Yemenis, Syrians and Saudis -- but Obeidi believes that has changed. "The Iraqi way of thinking in the past totally rejected that someone would kill himself," he said. "But once they realized how powerful this weapon is and saw its effectiveness, Iraqis started getting involved in suicide operations."

Some U.S. officials agree. "There's a kind of axiom out there that says Iraqis aren't suicide bombers," Gen. George W. Casey, commander of multinational forces in Iraq, told reporters in Baghdad this year. "I'm not sure that's the case. I believe there are Iraqi Islamic extremists ... that are very capable of getting into cars and blowing themselves up."

Other U.S. officials say they still believe that foreign fighters are responsible for most of the suicide attacks, which have increasingly targeted Iraqi civilians and security forces. "There is no evidence this is being done by Iraqis," said U.S. Maj. Gen. John Defreitas, intelligence chief for the multinational mission that has 150,000 troops in Iraq. "In every case we've seen, the driver has been a foreigner."

Coalition officials acknowledge, however, that the numbers show an Iraqi-dominated insurgency. Fewer than 5 percent of those killed or captured have been foreigners, one official noted. He also described the influx from abroad as a "very, very small part" of the estimated 12,000 to 20,000 insurgents.

A recent attack in the city of Baqubah illustrated an Iraqi role in suicide bombings.

While inspecting his security unit outside the Baqubah courthouse, Imad Shakir, a police major, observed an unfamiliar young man in an ill-fitting police uniform approaching.

Shakir's officers asked the purported first lieutenant for identification, then Shakir suddenly realized why he couldn't place him, the officers say. He leaped to seize the intruder but was too late to prevent the impostor from detonating his vest of explosives, lumpy and bulging beneath his blue clothing. Shakir, the suicide bomber and three bystanders died in the fiery May 15 explosion.

What set the Baqubah bombing apart from the few others in which survivors got a glimpse of the attacker was that Shakir's killer was recognizably Iraqi, said the Diyala province police lieutenant colonel in charge of the investigation. "The injured people assured us that the suicide attacker was Iraqi. They could tell by the way he talked and from his appearance," said the officer, who spoke on condition that he not be identified.

Obeidi, the retired general, sees the rise in suicide bombings as recognition among Iraqi extremists that they are an effective weapon against the superior numbers and arms of the occupying forces.