KABUL, Dec 23 (Reuters) - America's top military officer announced a near-doubling of U.S. troops in Afghanistan at the weekend, and then flew east to cool tensions between Pakistan and India. The reason? It's all part of the same security equation.

If a "surge" of up to 30,000 extra soldiers to Afghanistan by next summer is the tactic chosen to beat the Taliban insurgency there, holding India and Pakistan back from each others' throats is the strategy to ensure peace across the region as a whole.

"The surge is not an answer by itself," said Brahma Chellaney, professor of strategic studies at the Centre for Policy Research in New Delhi. "It's not the number of troops, it's the strategy."

Last month's attacks by suspected Pakistan-based militants in the Indian city of Mumbai, in which 179 people died, has halted the faltering peace process between New Delhi and Islamabad. While neither side has moved to a war footing, the prospect of a conflict must be concentrating minds in the Pentagon and NATO.

For one thing, there is a risk Pakistan would move some of the nearly 100,000 troops it has on its western border, with Afghanistan, to reinforce security along its frontier with India, with which it has already fought three wars since 1947.

That would take pressure off Taliban fighters who hide in the borderlands planning attacks on Western forces in Afghanistan.

"High tension between Pakistan and India doesn't serve American interests. It undermines America's agenda to control terrorism and that will only succeed if India-Pakistan relations are normalised," said Professor Hasaan Askari Rizvi, a Pakistani academic and political analyst.

"Pakistan's attention has now been diverted from the tribal areas to the eastern border which means the Taliban and other militant groups now have greater freedom and that means they can engage in more activities."

Vanda Felbab-Brown, a Brookings Institution security expert, said Pakistan had stepped up the fight against militants, but not enough, and may lose the will to press on because of Mumbai.

"Our hand is going to be weaker now due to Pakistan's tensions with India," she said. "It's quite possible that even the willingness they have generated up to now they will lose for the next two months."

FEAR OF ENCIRCLEMENT

Afghan officials often accuse elements within Pakistan's ISI spy agency of secretly supporting Taliban insurgents.

Pakistan denies the charge, but analysts say many of its military brass are suspicious of the ties between Afghanistan and India and fear encirclement with a hostile India on its eastern flank and hostile Afghan forces, backed by New Delhi, on the west.

Increased tension with India would only reinforce those who argue that the Taliban are a useful foreign policy tool.

"Some (but not all) in the establishment see armed militants within Pakistan as a threat -- but they largely consider it one that is ultimately controllable, and in any case secondary to the threat posed by their nuclear-armed enemies," wrote Barnett Rubin and Ahmed Rashid in the Foreign Affairs journal.

The commander of international forces in Afghanistan had actually sought extra troops before the Mumbai attacks, and long before U.S. Joint Chiefs of Staff Chairman Admiral Mike Mullen announced on Saturday that 20,000-30,000 more would be deployed.

So perhaps the "surge" would have happened anyway.

However, it was not by chance that Mullen flew to Islamabad two days later to lecture military chiefs there on the importance of joining hands with India "to combat ... extremism together".

U.S. President-elect Barack Obama has made it clear he will adopt a dual strategy of maintaining peace between India and Pakistan and beating the Taliban, who remain a formidable force seven years after U.S.-led forces ousted them from power.

"Obama has also been clear that he sees the Pakistan situation very much through the prism of India," said a NATO diplomat, who asked not to be named.

"So I think you see the whole U.S. administration taking a much wider regional focus, which is very very valuable." (Additional reporting by Robert Birsel and Kamran Haider in Islamabad, Paul Tait in Delhi, Paul Eckert in Washington and David Brunnstrom in Brussels; Editing by John Chalmers) ((Kabul newsroom, +93 708 871 211))