By Missy Ryan and Susan Cornwell
WASHINGTON (Reuters) - Support is growing in the U.S. Congress for expanding American military action in Pakistan beyond drone strikes already used to target militants in Pakistani territory, a senior Republican U.S. senator says.
The comments by Senator Lindsey Graham, an influential Republican voice on foreign policy and military affairs, follow remarks by the top U.S. military officer, Admiral Mike Mullen, accusing Pakistan last week of supporting the militant Haqqani network's September 13 attack on the U.S. embassy in Kabul.
Graham said in an interview on Tuesday that U.S. lawmakers might support military options beyond the drone strikes that have been going on for years inside Pakistani territory.
Those options may include using U.S. bomber planes within Pakistan.
The South Carolina Republican said he did not advocate sending U.S. ground troops into Pakistan.
"I would say when it comes to defending American troops, you don't want to limit yourself," Graham said. "This is not a boots-on-the-ground engagement -- I'm not talking about that, but we have a lot of assets beyond drones."
"A perfect world ... would be Afghan, Pakistan and (U.S. and NATO) coalition forces working jointly on both sides of border to deny safe havens, inside of Afghanistan and on the other side," in Pakistan's western tribal regions from which the Haqqani network and other militants are believed to operate, Graham said.
Graham said U.S. lawmakers will think about stepping up the military pressure. "If people believe it's gotten to the point that that is the only way really to protect our interests I think there would be a lot of support," Graham said.
The Haqqani network is allied with Afghanistan's Taliban and is believed to have close links to al Qaeda. It fights U.S. and NATO forces in eastern Afghanistan, operating out of bases in Pakistan's North Waziristan.
U.S. drone aircraft in recent years have targeted mostly al Qaeda figures rather than Haqqani militants.
Increased U.S. military action on Pakistani soil, including the idea of U.S. soldiers crossing the porous border from Afghanistan, would be deeply unpopular in Pakistan. Pakistan viewed the U.S. military raid in May that killed al Qaeda chief Osama in Laden in a Pakistani garrison town as a grievous breach of its sovereignty.
"Don't underestimate how we feel about those who try to kill our troops," Graham said in the interview.
"My belief is that Congress will be supportive of any action that the (U.S. military) experts deem necessary to protect lives of American soldiers" in Afghanistan.
The tense ties between Pakistan and the United States worsened last week after Mullen, the chairman of the U.S. military's Joint Chiefs of Staff, described the Haqqani network as a "veritable arm" of Pakistan's ISI spy agency.
Graham, known as a hawk, said on Sunday that the United States must consider all options "including defending our troops" in confronting Pakistani support for militant networks active in Afghanistan.
Such remarks from the U.S. Congress, where patience has worn thin with Pakistan, have intensified speculation that the United States might resort to another cross-border raid such as the one that killed bin Laden, intensify drone attacks in Pakistan's lawless tribal regions or send in bomber planes to attack militant hide-outs.
Lawmakers are proposing to restrict U.S. aid to Pakistan by placing more rigorous conditions under which Pakistan, which possesses nuclear arms but is desperately poor, can access American military and economic assistance.
The United States has been frustrated by what it sees as Pakistan's unwillingness to stamp out militants like the Haqqanis and the Taliban in Afghanistan, where U.S. forces have been engaged in a war for the past decade.
The unusually public criticism from Washington has provoked anger among Pakistani leaders who warn that the United States may lose a key ally in an unstable region.
(Editing by Warren Strobel and Will Dunham)