While there are no active U.S. military plans for airstrikes against al-Nusra targets in Syria, neither the Pentagon nor the White House has rejected that idea as a possible future option, a senior U.S. official tells CNN.

“Nothing is off the table completely,” the official said.

The possibility of strikes against al-Nusra — the al Qaeda affiliate in Syria — was first reported by the Washington Post.

Senior U.S. military officials are making it clear the Pentagon is not anxious to undertake such strikes, knowing it would legitimately be seen as so-called mission creep, the official said. When asked by CNN’s Jim Acosta, White House press secretary Josh Earnest seemed to leave the door open, however.

“The United States remains concerned about all threats emanating from Syria, including the threat that’s posed from by the Nusra Front. That’s driven by the fact that … the Nusra Front has been public in threatening the West,” Earnest said Tuesday. “But what we will continue to do here in the United States is to work closely with our coalition partners to take strikes in Syria that are focused principally on denying a safe haven to those extremists and organizations that are seeking to do harm to the United States of America or our allies.”

Striking al-Nusra would have the goal of pushing back its advances against moderate Syrian rebels who the United States is backing in the fight against ISIS. The Pentagon has repeatedly said it sees the U.S. military priority as striking ISIS in both Syria and Iraq with the goal of ensuring Iraq does not fall to ISIS control.

But getting Syrian opposition into fighting shape to challenge ISIS has been a slow and complicated process due to the need to verify the security credentials of Syrian fighters going through the program. More than four months after announcing an effort to train and equip the moderate Syrian opposition, the U.S. military has yet to vet a single Syrian rebel to fight ISIS on the ground.

“There’s progress in setting up curriculum, there’s progress in getting the sites ready, there’s progress in getting trainers contributed to the effort — not just from the United States but from other nations,” Pentagon spokesman Rear Adm. John Kirby said Tuesday. “But the vetting progress has not begun yet.

Kirby added that the program “has not been completely codified,” but did not go into detail on how or when that process might unfold.

The idea of mounting a train-and-equip program for the moderate opposition was first floated over a year ago, in the aftermath of revelations that the Assad regime used chemical weapons against Syrian civilians.

But it was not until June of this year, when the terror group ISIS started making significant territorial gains in Iraq, that the Obama administration backed the plan, asking Congress for $500 million to “train and equip appropriately vetted elements of the moderate Syrian armed opposition.”

Prior to that, the U.S. had been providing nonlethal material and light weapons to the moderate opposition.

The effort is part of a $1.5 billion initiative the United States is undertaking with a number of regional partners.

Kirby also told reporters that the U.S. military is making significant progress in its overall efforts to degrade and destroy ISIS.

In particular, Kirby said, the Iraqi army is making meaningful territorial gains with help from U.S. advisers, and U.S.-led efforts are helping disrupt the group’s revenue and degraded its ability to maneuver, communicate and operate inside Iraq.

“Nobody is saying that it’s over. There’s a long way to go here,” Kirby cautioned. “But they’ve definitely felt … the full weight of the pressure that’s being put on them.”