The former U.S. ambassador to Afghanistan on Sunday said President Trump is more focused on Pakistan than his last two predecessors.

“I don't think either President Bush or President Obama focused as sharply, as clearly, that the sanctuary issue, Pakistan's both role as a facilitator of and a help to Afghanistan and as a sanctuary for those who fight us and in exchange, receive a lot of assistance, is not the number,” Zalmay Khalilzad, the ambassador to Afghanistan under President Bush, said on ABC's "This Week."

“This has been, in my judgment, the single most important factor, the Pakistan problem, for prolonging the war ... the sanctuary issues.”

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Trump's new policy on Afghanistan, announced last week, includes a plan to pressure Pakistan into taking a harder line on terrorist groups. Trump accused the country of providing safe havens to such groups.

Khalilzad said the United States has leverage over Pakistan it has not yet used, including cutting off assistance.

“We have the leverage of putting those individuals, as we do it in the case of ... Russia — or we do it in the case of Iran, to put individuals who support groups such as the Taliban, terrorist groups like Haqqani, on a blacklist,” he said.

Bush and Obama also tried to pressure Pakistan into taking a harder line on terrorist groups, but their actions did not yield significant results, thanks in part to fears of alienating a major non-NATO ally.

Pakistan denies that it provides safe haven to terrorists.