BAGHDAD, Iraq — Suicide bombings have become the Iraqi insurgency's weapon of choice, with 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, handover of sovereignty.

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

With U.S.-led forces 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 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 said 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 target U.S. and Iraqi forces.

Suicide attacks are on the rise because the bombs "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 that 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.

Saad Obeidi, a retired Iraqi major general and security expert, suggested President Bush had invited Islamic extremists to bring their fight against the United States to Iraq. "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."

The first suicide bombings of the insurgency were attributed to foreign infiltrators — mostly Palestinians, Yemenis, Syrians and Saudis — but Obeidi thinks that has changed.

"The Iraqi way of thinking in the past totally rejected that someone would kill himself," Obeidi said. "But once they realized how powerful this weapon is and saw its effectiveness, Iraqis started getting involved in suicide operations."

"There's a kind of axiom out there that says Iraqis aren't suicide bombers," Gen. George Casey, commander of multinational forces in Iraq, said 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 said they believe 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 acknowledged, 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 12,000 to 20,000 insurgents.

A recent attack in 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 man in an ill-fitting police uniform approaching.

Shakir's officers asked the purported first lieutenant for identification and Shakir suddenly realized why he couldn't place him, the officers said. He leaped to seize the intruder but was too late to prevent the impostor from detonating his vest of explosives. 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 official in charge of the investigation.

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.

"Fighters are choosing this method to create a balance against superpower might," he said, referring to U.S. forces.

Maj. Gen. Munem Said Abdulqadir, head of the Iraqi police force explosive ordnance demolition team in Baghdad, blames the now-disbanded Coalition Provisional Authority for barring even midlevel figures of Saddam Hussein's government from the new security order.

He said he feared there are thousands of technically savvy and disaffected Iraqis, mostly Sunni Arabs, at large and vulnerable to recruitment as suicide bombers. "Jobless people are very easy targets," he said. "Find them jobs, and most will give up."

The attackers show no sign of letting up.

A suicide car bomber attacked the main checkpoint to Baghdad's international airport yesterday, wounding 15 Iraqis, the U.S. military said.

Today, a car bomb targeting a northern Iraq restaurant where bodyguards of Iraq's Kurdish Deputy Prime Minister Rowsch Nouri Shaways were eating killed nine people and injured 25, said police Brig. Sarhad Qadre. Shaways was not hurt.

Also today, a car bombing killed the deputy head of Diyala provincial council and three of his bodyguards north of Baghdad, police said.

In Kirkuk, meanwhile, a suicide car bomber killed two Iraqi bystanders and wounded eight, police said.

Material from The Associated Press is included in this report.