A top Senate Republican on Thursday questioned whether President Obama's new Afghanistan strategy does enough to address Al Qaeda and Taliban safe havens in Pakistan, saying any plan will be "substantially incomplete" if terrorists are not flushed out across the border. 

Intelligence officials say that only about 100 Al Qaeda operatives remain in Afghanistan and that while they have great influence over the larger Taliban network, Al Qaeda's base has moved to Pakistan. Sen. Richard Lugar, R-Ind., ranking Republican on the Senate Foreign Relations Committee, called on the president's national security team to make clear how the new strategy will promote a stronger alliance with Pakistani forces, warning that Pakistan's nuclear weapons make it a riskier landscape than Afghanistan. 

"It is not clear how an expanded military effort in Afghanistan addresses the problem of Taliban and Al Qaeda safe havens across the border in Pakistan," he said. "If these safe havens persist, any strategy in Afghanistan will be substantially incomplete." 

Lugar spoke as Obama's security team faced lawmakers on Capitol Hill for the second day in a row to explain why 30,000 troops are being ordered to Afghanistan and under what conditions they will be brought home. 

Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, assured Lugar that Pakistan was a "critical part" of the three-month strategy-drafting process and agreed that the link between the trajectory of both countries is "almost absolute." 

"There was an enormous amount of time spent on Pakistan," he said, saying a "long-term partnership" is needed with the Pakistan government. "The outcome in Afghanistan bears directly on Pakistan's future." 

Secretary of State Hillary Clinton and Defense Secretary Robert Gates are also on the Hill. 

The team faced tough questions Wednesday over Obama's decision to choose July 2011 as the date when the United States will begin to transfer responsibility to the Afghans and withdraw from the country. Ultimately, Gates and others conceded that the date is a flexible target and could change if the Afghans are not ready to handle the insurgency on their own. 

Mullen told Fox News on Thursday that July 2011 is a "target date" for the administration. 

"We'll know at that point in time how well things are going or how poorly things are going and it is really a target date at which we will start to transition and transfer responsibility," he said. 

The surge-and-exit strategy that Obama announced Tuesday night marks the largest expansion of the war since it began eight years ago. 

Despite misgivings, members of Congress seem poised to back Obama's plan. Critics concede that Obama will have little trouble early next year getting Congress to provide an added $30 billion or $40 billion to carry it out. 

Republicans, despite questions about the withdrawal date, otherwise support the troop buildup because it fulfills a request by U.S. commanders in Afghanistan for more soldiers and Marines. 

Anti-war Democrats, who rose to power because of voter opposition to Bush's strategy in Iraq, said they are skeptical that the troop buildup is necessary or will work. But at the same time, party leaders -- who were among Obama's biggest supporters in his campaign for president -- said it was unlikely that they would try to block the deployments or the money he wants.

The Associated Press contributed to this report.