The chairman of the US Joint Chiefs of Staff, Admiral Mike Mullen, said he feared extremists would attempt another operation similar to the 2008 Mumbai attacks in order to goad India into armed retaliation against its neighbour and arch-rival.
Mullen said the Mumbai carnage, which India blamed on the Pakistan-based Lashkar-e-Toiba (LeT) militant outfit, had demonstrated how a small group of extremists could have a "strategic impact".
"One of the things that struck me then and is still of great concern is that those terrorists could bring two countries closer (to a possible conflict)," he told reporters on board his plane bound for New Delhi.
India suspended a peace dialogue with Pakistan in the wake of the Mumbai attacks, which claimed 166 lives, and the two countries have only recently begun to explore a resumption of structured talks.
"I've worried a great deal about a repeat attack, of something like that," Mullen said, adding that he wanted "to focus on making sure this doesn't happen again."
Mullen began a two-day visit to India on Thursday that coincided with a visit by the US special envoy to Pakistan and Afghanistan, Richard Holbrooke.
Speaking earlier to reporters in Delhi, Holbrooke said the LeT was just one of a number of regional militant groups, along with the Pakistani and Afghan Taliban, looking to destabilise South Asia.
"They seem to be growing closer together ... (and) their long-term objective is the same: to create the maximum number of problems between India and Pakistan ... to create a crisis," Holbrooke said.
Both men said combating the LeT was a top priority.
"I see them starting to emerge as a larger regional to global - at least aspirational - global threat," Mullen said.
India accuses Pakistan of failing to crack down sufficiently on groups like the LeT that operate from bases on its territory, and Indian officials were expected to highlight the same charge during talks with Holbrooke and Mullen.
US officials said they have pressed Pakistan to prosecute LeT extremists but have so far made little headway.
"We haven't had a breakthrough on the Mumbai issue," Vali Nasr, senior adviser to Holbrooke, told reporters at the US embassy.
Washington would continue to raise its concerns amid hope that improving US ties with Pakistan "will make these discussions more fruitful," he added.
Mullen's meetings in New Delhi are also expected to focus on military cooperation and conditions in Afghanistan, where India is concerned about what it perceives as growing Pakistani influence.
India and Pakistan have fought three wars since the division of the sub-continent in 1947 and their relationship has always been beset by mutual mistrust.
Holbrooke, who was due to depart Delhi for London, rejected the suggestion that India was somehow being sidelined by Pakistani involvement in the Afghan government's plans for reconciliation and reintegration of the Taliban and other groups.
"You cannot stabilise Afghanistan without the participation of Pakistan as a legitimate concerned party," he said.
"India is not being diminished. It's not a zero-sum game," he said. "India also has a major role to play in stability in the region and in search for solutions in Afghanistan."
At the same time, Holbrooke said Washington had also raised concerns with Islamabad about links between Pakistan's ISI intelligence agency and the Taliban.
"The links between the ISI and the Taliban are a problem," he acknowledged.
Mullen, however, stressed that the Obama administration had no choice but to "stay engaged" with the ISI, given its crucial role in Pakistan.
While Washington recognised the ISI helped provide for Pakistan's security, Mullen said, "We differ on the specifics on how that should be done."
The admiral was due to head to Pakistan after his two-day stop in India.
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