Al-Qaeda in Iraq is rigging houses and shops with explosives in a new tactic that has killed and maimed civilians in recent weeks and defied the thousands of security forces in Baghdad, officials say.

The renting of residential buildings for targeted bombings has forced police and the army to adapt their operations, in a bid to prevent more of the attacks that have killed dozens since the country's inconclusive March 7 election.

The US military has even coined a new acronym -- HBIED (house-borne improvised explosive device) for the bombings, which have also left hundreds wounded in the past month in the Iraqi capital.

The HBIED follows the IED (improvised-explosive device -- homemade bomb) and VBIED (vehicle-borne improvised-explosive device -- car bomb) into a terrorist lexicon started in Iraq and subsequently transported to Afghanistan.

"Our forces are focusing on the renting of apartments and buildings," Major General Qassim Atta, a Baghdad security forces spokesman, told AFP.

Insurgents were continually looking to exploit gaps in the city's defences, he said.

"They change their methods periodically because most of their plans and tactics have been discovered. I believe they are already searching for another method of attack, maybe churches or bridges."

Some 25 people were killed on election day, when explosives destroyed two buildings in northeast Baghdad. The US military, which pointed the finger at Al-Qaeda, said the properties had been rented and deliberately blown up.

A further 35 people died on April 6, when explosives were planted in houses and shops in mostly Shiite neighbourhoods, leading Atta to say Iraq was in an "open war" with Al-Qaeda and loyalists of executed dictator Saddam Hussein.

A number of those properties had also been rented days earlier, security officials told AFP.

Counter-terrorism experts say the insurgents are placing bombs in houses and shops despite the methods being frowned upon by much of Al-Qaeda.

"These stories are credible," said Brian Fishman, a counter-terrorism research fellow at the New America Foundation in Washington DC, and author of "Dysfunction and Decline: Lessons Learned from Inside Al-Qaeda in Iraq."

"The tactic is seen as very disreputable, even among active insurgents," but it allows them "to get around a lot of the tactics developed to prevent car bombs," such as the mass of security checkpoints in Baghdad, Fishman said.

"Al-Qaeda in Iraq's propaganda arm has disavowed the tactic and urged supporters to distribute their statement widely ... but that doesn't mean they are not doing it," he added.

American officers agree that insurgents, including Al-Qaeda, have adapted their tactics.

"The terrorist threat that exists in this country, and it does exist to a degree, will continue to evolve," said US military spokesman Brigadier General Steve Lanza.

"One of the tactics you have seen is to take buildings and to destroy them, causing a lot of collateral damage and a lot of injuries. The purpose is to foment sectarian violence but they have not succeeded."

A series of massive bombings in Baghdad since last August, including several attacks on government ministries, which killed more than 400 people, has undermined confidence in Iraq's security forces.

The lack of a clear election winner has also raised fears of a vacuum that could be exploited by insurgents ahead of a pullout of US combat troops in August, followed by a complete military withdrawal at the end of 2011.

An Iraqi military officer, speaking on condition of anonymity, said would-be bombers attempted to capitalise on any weakness in the war-torn country's security apparatus. Checkpoints were an inadequate defence, he admitted.

"The streets cannot be controlled," the officer said.

"When the army and police have doubts about a car, they search the trunk which is seen by the naked eye but they ignore many other parts of the car that may be filled with explosives."