The United Nations, meanwhile, denounced war crimes committed both by Gaddafi's forces and the rebels vying to oust the Libyan strongman.
"There are from my perspective some signs, certainly in the last few days, that Gaddafi is becoming more and more isolated," Mullen told reporters in Washington.
Mullen pointed to the defection of oil minister Shukri Ghanem, who had been a key figure in the regime, along with a group of "young generals" who had also parted with Gaddafi.
He also welcomed NATO's extension of its UN-mandated mission to protect civilians through military action until late September.
"I think from my perspective, and I've engaged with the commanders on this, that we''re going to be okay until September," he said.
An AFP correspondent said a series of blasts overnight shook the Libyan capital, the target of intensive NATO air raids in the past few weeks.
In its latest operational update released on Thursday, NATO said its jets had bombed a vehicle store and surface-to-air missile launcher in the vicinity of Tripoli.
Earlier, NATO Secretary General Anders Fogh Rasmussen told reporters that Gaddafi's departure was only a question of time.