BAGHDAD (AFP) – As a joint operation by US and Iraqi troops to take control of Baghdad begins to bear fruit, there were signs on Thursday that their insurgent foes are trying to counter them with deadly new tactics.

Iraqi medics were treating patients poisoned by what is thought to be chlorine gas after attackers targeted civilian areas with trucks rigged as dirty bombs, said Qais Abdulwahab, director of the Kadhimiya Hospital.

Meanwhile, US commanders are investigating the loss of the latest in a series of helicopters after a Blackhawk came down on Wednesday in fields north of the capital after coming under fire from the ground.

“Operation Fhard al-Qanoon” (Imposing Order) has scored some successes. Murders are down in Baghdad and more than 90,000 US and Iraqi troops have met only token resistance as they fan out through flashpoint districts.

Three suspected Al-Qaeda insurgents were killed north of Baghdad in clashes on Thursday and overnight raids in the city netted five “rogue” members of the Shiite Mahdi Army militia suspected of kidnapping and murder.

But daily bomb attacks on civilians continue, and the use of chlorine and anti-aircraft tactics has underlined what US commanders say is the insurgents’ main strength — their ability to adapt and exploit their foes’ weaknesses.

“One of the things we see as we deal with this is that as one technique works in one part of the country we tend to see copycat attacks in other parts of the country,” said US spokesman Lieutenant Colonel Chris Garver.

On Tuesday, a truck carrying chlorine gas exploded in Taji, just north of Baghdad, killing six people on the spot but also poisoning scores more as the toxic gas spread through the area, overcoming women and children.

On Wednesday, the dirty bombers struck again, in the suburbs of Baghdad, in a less successful attack that nevertheless spread panic.

“The material used is poisonous,” said Abdulwahab. “During the explosion it changes into a mist that spreads through the air, causing poisoning in the breathing system, breathing difficulties and acute coughing.

“It’s is the first time we have seen such poisoning cases,” he told AFP, comparing the injuries to the internal burns suffered by children who drink chlorine-based cleaning products.

Kadhimiya Hospital treated 90 patients poisoned in Tuesday’s attack — seven of whom died — and 21 more on Wednesday, Abdulwahab said.

The Martyr Mohammed Baqir al-Hakim Hospital in Shula treated 66 cases after the Taji blast, and all survived, said medical official Abu Murtadha.

“They’ve adapted the car bomb tactic,” Garver said. “It shows some of the maliciousness with with they are adapting those tactics.

“It was not a chlorine tanker it was just a tank in the back of a truck. The use of canisters with something in them is not new, they’ve tried using regular acetylene tanks to increase the size of the explosive,” he said.

“So that’s not new, we do look for canisters already, but obviously we are going to pay more attention now to any kind of canister,” he said.

Meanwhile, US commanders are examining the threat to their helicopters, eight of which — two operated by a private security outfit — have been lost since January 20, most of them shot down by insurgents.

The latest was a Blackhawk transport that made a “hard landing” north of Baghdad late Wednesday. The nine personnel on board survived, but the incident underlined the choppers’ apparently increasing vulnerability.

“Initial indications appear that it was brought down by small-arms fire and rocket-propelled grenades,” Garver said.

The Blackhawk was the third of the helicopters hit in a region north of Baghdad. Insurgents linked to Al Qaeda claim in Internet messages to have deployed “new tactics and weapons” in the area.

“We are engaged with a thinking enemy,” warned US Major General James Simmons, who oversees helicopter operations in Iraq, last week.

“This enemy understands that we are in the process of executing the (Iraqi) prime minister’s new plan for the security of Baghdad, and they understand the strategic implications of shooting down an aircraft,” he said.

As America’s enemies in Iraq — Sunni insurgents and Shiite militiamen — have become more adept in their use of roadside bombs, the military has made greater use of helicopters.

In 2004, US army helicopters flew 240,000 hours in Iraq, and in 2006 334,000. This year they are expected to fly at least 400,000 hours. Simmons insisted it is still the safest way to get about.

The United States has decided to boost its troop levels in Iraq by 21,500 by the end of May, but on Wednesday coalition ally Britain announced that it would scale back its forces and Denmark and Lithuania said they were pulling out.