Marine General Joseph Dunford, chairman of the Joint Chiefs of Staff, said, "Pakistan is a key to Afghanistan and its security."
"And ensuring that Haqqani does not have sanctuary in South Asia, and making sure the Taliban don't have sanctuary in South Asia, making sure there's a secure border between Afghanistan and Pakistan is critical, making sure there's a effective political and military relationships between Pakistan and Afghanistan, that's one of the interdependent variables that's going to allow us to be successful," he said.
Gen Dunford, however, said that "no decision" has been made yet on US troop numbers in Afghanistan.
US Defence Secretary James Mattis last week said President Donald Trump has authorised him to "manage" troop levels in Afghanistan. That confirmation was seen as an indication that more American forces could be sent to the war-torn country amidst a surge in attacks by the Taliban.
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"One decision that was made by the president was to delegate that decision to Secretary Mattis in terms of forces that would be on the ground," he said.
"So when Secretary Mattis makes a decision about force levels...You can expect that he'll communicate that in a broader context; again, specifically the context of that strategy review. So, it won't be just about Afghanistan. There are a number of interdependent variables that bear on the problem inside of Afghanistan across the region. And we'll be prepared to talk about those as well when we talk about force management levels," Dunford said.
"In other words, he's identified areas where he believes additional forces could make the adviser effort in Afghanistan more effective," Dunford said.
"There is also an outstanding requirement for forces that the commander asks for from NATO last year. So that's what you also heard him talk about publicly. And we're short about 3,000 from the stated NATO requirement for forces in Afghanistan. That's where the numbers come from, but again, what I'd emphasise is that, any decision on numbers is going to be done in that broader context," he said.