WASHINGTON (Reuters/Tim Reid) - Pakistan's Washington lobbyists have launched an intense campaign on Capitol Hill to counter accusations that Islamabad was complicit in giving refuge to Osama bin Laden.

Alarmed by lawmakers' demands to cut off billions of dollars of U.S. aid after bin Laden was found living in a Pakistani safe house for six years, President Asif Ali Zardari has ordered a full-court press to quell mounting accusations that it helped the al Qaeda leader avoid capture.

Mark Siegel, a partner in the Washington lobbying firm of Locke Lord Strategies -- which is paid $75,000 a month by the Pakistani government -- told Reuters on Thursday he had spoken twice to Zardari since U.S. special forces killed bin Laden on Sunday, and "countless" times to the Pakistani ambassador in Washington.

"They are certainly concerned," Siegel said, adding that suggestions the Pakistani government knew about bin Laden's whereabouts was nothing more than speculation.

Referring to a statement by President Barack Obama's counterterrorism adviser, John Brennan, that there must have been a support system for bin Laden inside Pakistan, Siegel said: "There is no proof that a support system was government-based."

There is much at stake for Pakistan as many lawmakers question how bin Laden could have lived in a large fortified compound close to a Pakistani military base for so long.

Some members of Congress are now demanding that nearly $3 billion in annual aid for Pakistan, included in Obama's 2012 budget, be blocked until the Zardari administration explains how bin Laden lived untouched just 30 miles outside Islamabad, the Pakistani capital. Pakistan has received over $20 billion in U.S. aid since the September 11, 2001, attacks.

Patrick Leahy, the Democratic chairman of the Senate subcommittee that allocates foreign aid, said on Thursday he wants a complete review of U.S. aid to Pakistan.

Leahy said he was certain that some Pakistani military and intelligence officials knew that bin Laden was hiding so close to Islamabad.

"It's impossible for them not to have some idea he was there," Leahy told Vermont Public Radio.

But Siegel, referring to claims by the Afghan government that Pakistan must have known bin Laden's whereabouts, said: "Must have known doesn't mean knew."

Siegel's firm was retained by the Zardari government in 2008 and has earned nearly $2 million in fees since then, according to Justice Department records. Siegel said his firm is paid $900,000 a year by Pakistan.

Since bin Laden's death, Siegel says he has been on Capitol Hill every day to promote Pakistan's position on the bin Laden killing, talking to congressmen, senators and their aides.

(Editing by Xavier Briand)

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