By Raheem Salman and Yara Bayoumy

BAGHDAD (Reuters) - Islamic State militants are changing tactics in the face of U.S. air strikes in northern Iraq, ditching conspicuous convoys in favor of motorcycles and planting their black flags on civilian homes, tribal sources and eyewitnesses say.

They reported fewer militant checkpoints to weed out "apostates" and less cell phone use since the air strikes intensified and more U.S. allies pledged to join the campaign that began in August, saying the militants had also split up to limit casualties.

A tribal sheikh from a village south of Kirkuk said Islamic State elements "abandoned one of their biggest headquarters in the village" when they heard the air strike campaign was likely to target their area.

"They took all their furniture, vehicles and weapons. Then they planted roadside bombs and destroyed the headquarters," said the tribal sheikh who declined to be named.

"They don't move in military convoys like before. Instead they use motorcycles, bicycles, and if necessary, they use camouflaged cars," he said.

The militants have also taken to erecting their notorious black flag on the rooftops of several mostly empty residential houses and buildings, to create confusion about their actual presence.

Civilian casualties are a major concern as U.S. war planes venture deeper into the Tigris River valley and to Iraq's western desert in the name of breaking Islamic State's grip on mostly Sunni parts of Iraq -- nearly one-third of the country. France has also taken part in the air campaign.

Tribal and local intelligence sources said an air strike on Thursday near Bashir town, 20 km (12 miles) south of Kirkuk, had killed two local senior Islamic State leaders while they were receiving a group of militants from Syria and Mosul. Ongoing fighting makes it impossible to verify the reports.

EXECUTING PEOPLE LIKE DRINKING WATER

A U.S.-led coalition has started bombing the militants in Syria as well, fearing the Sunni extremist group could threaten national security from a caliphate they have declared in territory seized there and through the border into Iraq.

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Arab allies have joined in and this week Denmark and Britain both pledged fighter jets to Iraq but not Syria.

In another village near Haweeja in northern Iraq, a source said the militants had ditched the use of long convoys of conspicuous vehicles with mounted machine guns and also noted their new preference for motor-bikes.

Islamic State fighters, who have controlled much of Syria's eastern oil and agricultural provinces for more than a year, swept through mainly Sunni Muslim regions of north Iraq in mid-June, seizing cities including Mosul and Tikrit and halting less than 100 miles (160 km) from the capital Baghdad.

But their recent moves suggest they are worried about the air strikes, which are backed on the ground by a largely hapless Iraqi army but a more formidable Kurdish peshmerga force.

"They were executing people like drinking water ... Now the air strikes are very active and have decreased the (militants') ability," Sheikh Anwar al-Assy al-Obeidi, the head of his tribe in Kirkuk and across Iraq, told Reuters.

"Wherever they hide, people want to get rid of them because they're afraid their houses will be struck," said Obeidi, who fled to Kurdistan this summer after Islamic State blew up his home.

SHUTTING CELLPHONES, SWITCHING CARS

The insurgents have gone underground in their main Syrian stronghold since U.S. President Barack Obama authorized U.S. air strikes on the group in Syria - which began earlier this week.

They have disappeared from streets, redeployed weapons and fighters, and cut down their media exposure, residents said.

The air strikes have by no means crippled them. Their fighters edged towards a strategic town on northern Syria's border with Turkey on Friday, battling Kurdish forces, while air strikes hit their oilfields and bases in Syria's east.

In the eastern Iraqi province of Diyala, an eyewitness said the air strikes had forced the militants to cut back the number of checkpoints which inspected identity cards, looking for those they considered "apostates": Shi'ites, policemen, soldiers.

"They have also increased the number of headquarters, instead of two, they now have 20, with only 3-4 people in each one of them," said the eyewitness.

An eyewitness in Jalawla town in Diyala also said the militants had decreased their presence on the frontline, no longer confronting army troops with large numbers.

In Tikrit, police colonel Hassan al-Jabouri said the militants had withdrawn their checkpoints from main thoroughfares in the city, retreating to side streets.

"They have also switched cars between the areas they control and our intelligence indicates that they have all changed their cell phones. These are always shut and the batteries are removed unless they need to use them," Jabouri told Reuters.

In perhaps the most obvious indication the militants are wary of the strikes, they have taken to digging and hiding in trenches -- just big enough for two people -- in residents' backyards.

(Additional reporting by Ned Parker; writing by Yara Bayoumy; editing by Philippa Fletcher)