LONDON, Dec 1 (Reuters) - Rising tension between India and Pakistan over the Mumbai attacks could set back Pakistan's offensive against Islamist militants along the Afghan border, Britain's top military officer warned on Monday.

Air Chief Marshal Jock Stirrup said he had been encouraged in recent weeks by Pakistan's approach towards insurgents in the semi-autonomous tribal areas bordering Afghanistan.

But Stirrup, chief of the defence staff, said last week's attacks in Mumbai, when 10 Islamist gunmen killed 183 people, "could set us all back considerably."

"If tensions between India and Pakistan continue to escalate, there's a risk they and we could be diverted from the real issue: Dealing with the terrorist groups who perpetrate such criminal and barbaric acts," he said in a speech to the Royal United Services Institute, a defence thinktank.

Indian investigators said on Monday the attackers had months of commando training in Pakistan. A day earlier, India said it was raising security to a "war level".

New Delhi has not accused Islamabad's civilian government of involvement, but it has expressed deep frustration that its neighbour has been unable or unwilling to prevent militants using its soil to attack Indian cities.

"The nationality of the terrorists does not strike me as the key issue," Stirrup said.

"The key is whether those countries work wholeheartedly with others to eliminate such terrorism as a force in international affairs."

"Nothing would warm the terrorists' hearts more than knowing that they'd succeeded in setting state against state," he said.

U.S. officials have long said the Taliban, al Qaeda and other militant groups operate safe havens on Pakistan's side of the border with Afghanistan and that these have been used to launch attacks on U.S. and NATO forces in eastern Afghanistan.

Stirrup said Britain was close to the stage where it would be able to fundamentally alter its mission in Iraq, dramatically reducing its 4,000 troops there.

He said there could be no "one-for-one" transfer of British troops to Afghanistan from Iraq, but did not rule out increasing the British contingent in Afghanistan. (Reporting by Adrian Croft; Editing by)

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