Al-Qaida militants are poised to rise again in Afghanistan after US forces pull out in December, political analysts have warned.
Al-Qaida's leader in Afghanistan, Farouq al-Qahtani al-Qatari, has been building up training of small fighting units, according to reports.
US drone and missiles attacks have targeted training camps, while President Obama has ordered the Pentagon to start preparing for the complete withdrawal of troops from Afghanistan in December.
He had offered to maintain a reduced US military presence in Afghanistan after December but was turned down by Afghan leader Hamid Karzai.
In November, Karzai agreed to let some US troops stay but would not sign a document to that effect until after the presidential election in April.
Failing to finalise the deal would "prevent the United States and our allies from being able to plan for a post-2014 presence in Afghanistan", White House spokesman Josh Earnest said in November.
It would also compromise counter-terrorism measures, he added.
Congress had threatened to cut off aid to Afghanistan if US troops were not allowed to stay.
"We have made clear that our commitment to Afghanistan – separate from the troop presence – is in our national security interests," a White House spokeperson said.
The Obama administration would like to leave up to 10,000 troops out of 38,000 in place after combat operations end. The troops would stay to train Afghan forces and combat terrorism.
The White House also warned that even if the security agreement were signed imminently, the number of US troops was in doubt.
House Intelligence Committee chairman Mike Rogers said the number of al-Qaida members in Afghanistan had slightly risen.
"Most [ of al-Qaida members] are waiting for the US to fully pull out by 2014," he warned.
US military and intelligence officials said that unless they could continue to fly drones and jets from at least one airbase in Afghanistan, Qahtani and his followers could eventually plan new attacks against US targets.
After talks with Karzai on Tuesday, Obama ordered the Pentagon to begin planning for the so-called "zero option", or full withdrawal.