WASHINGTON - The United States remains concerned about India-Pakistan tensions as the nuclear-armed countries' militaries remain on alert nearly three weeks after their most dangerous confrontation in decades, a senior U.S. administration official said on Wednesday.The official also indicated that the Trump administration does not think Pakistan has adequately cracked down on the Islamist extremists who claimed responsibility for a bomb attack on Indian security forces that triggered last month's crisis."If there is an additional terrorist attack without Pakistan having made a sustained sincere effort against these groups, it will be extremely problematic for Pakistan and it would cause a re-escalation in tensions," the official, who briefed reporters on condition of anonymity.The official's comments underscored the Trump administration's view that Pakistan's harboring of extremist groups lies at the heart of the latest upsurge in tensions.The crisis erupted with a Feb. 14 suicide bombing on India's side of the disputed Kashmir region that killed 40 paramilitary officers and was claimed by Jaish-e-Mohammed, a militant group that US officials charge is harbored by Pakistan. Islamabad denies the allegation.On Feb. 26, India launched its first-ever airstrikes on an alleged extremist training camp in Pakistan. The following day, Pakistani aircraft retaliated against targets inside India-controlled Kashmir, triggering a dogfight.In their first such clash since a 1971 war, Pakistan downed an Indian plane and captured its pilot after he ejected.The Trump administration, backed by other powers, pressed the sides to avert further violence amid fears of an all-out war that could go nuclear.While the sides have taken steps to de-escalate tensions, including Pakistan's return of the Indian pilot, the U.S. official said that Washington remains concerned."We do still see the militaries on alert and so we realize if there, God forbid, would be another terrorist attack, then you could quickly see escalation in the situation once again," the official said. "We are making clear that any additional military action by either side runs an unacceptably high risk for both countries and for the region."Pakistan says it arrested dozens of extremists and seized their assets. But the official indicated that Washington does not believe the crackdown has been sufficient."I think we will need to see irreversible sustained action. It's early to make a full assessment," the official said.Competing claims to Kashmir by India and Pakistan make the area one of the world's most dangerous flashpoints.