WASHINGTON, Dec 1 (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.

Seven years after U.S.-backed forces toppled the Taliban government in Afghanistan, the United States still has about 32,000 troops in the country trying to quell a big rise in violence by Taliban militants and other insurgents.

The United States has pressured Pakistan to do more to control the border and combat militants believed to exploit safe haven in Pakistan to launch attacks in both countries.

A redeployment of Pakistani troops to the Indian border could undercut that effort.

POLITICAL INSTABILITY IN PAKISTAN

To avert a crisis, U.S. Secretary of State Condoleezza Rice telephoned Pakistani President Asif Zardari last week to urge him to make clear his government was not behind the Mumbai attacks and to cooperate fully in the investigation.

"Follow every lead no matter where it goes -- that is what we want to see them do," said a U.S. official.

The official would not elaborate, but he appeared to be alluding to the possibility that members of Pakistan's main Inter-Services Intelligence security agency might have a link to the militants behind the violence.

Pakistan has vowed to cooperate with India in investigating the attack and it has rejected what it called unsubstantiated allegations of complicity in the violence. However, it has backtracked on a decision to send the chief of the ISI.

Reform of the ISI has long been a goal of the United States but the Pakistani government's effort to place it under Interior Ministry control this year was quickly reversed.

Another U.S. official sketched out what he called a "worst case scenario" under which Zardari might seek to reform the ISI, the agency would resist and Pakistani public opinion might run against him, eroding his political standing.

"If the ISI plays hardball ... and Zardari is viewed with suspicion and as siding with the Indians, (it is possible) he begins to weaken, to lose authority, and we wind up with another political crisis," he said. (Editing by Mohammad Zargham)

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