SCENARIOS - Mumbai attacks open South Asia Pandora's box for U.S.

WASHINGTON (Reuters) - U.S. officials fear the deadly Mumbai attacks could worsen Indo-Pakistani tensions and prompt Pakistan to move forces from its Afghan border toward India, undermining U.S. counterterrorism efforts in Afghanistan.

Investigators have said that the attacks, which killed 183 people in India's financial hub, were carried out by militants trained by the Pakistan-based Lashkar-e-Taiba group, which is blamed for a 2001 attack on India's parliament.

That attack nearly sparked a fourth war between India and Pakistan, both of which have nuclear weapons. Below are some of the dangers that U.S. officials said they perceive after the latest violence and how they are trying to avoid them.

TROOP SHIFT FROM AFGHAN BORDER

U.S. officials said their top concern would be heightened tensions between India and Pakistan, which have fought three wars since their 1947 independence from Britain.

The two countries massed a million soldiers on their border after the 2001 attack on the Indian parliament, triggering an aggressive crisis management effort by then U.S. Secretary of State Colin Powell and his deputy, Richard Armitage.

"What we are also very concerned about is any move by the Pakistanis to take troops away from the western front and move them to the eastern border with India," said a U.S. official who asked not to be named.

Officials in Pakistan have said that if tension with India escalates, they would have to move troops from its Afghan border, where it is battling al Qaeda and Taliban fighters responsible for violence in Pakistan and Afghanistan.