WASHINGTON - The United States' military footprint in Afghanistan is shrinking at a steady pace, according to U.S. military planners, though they are refusing to say how many troops are still in the country.

U.S. Central Command announced Tuesday that it has completed "between 6-12% of the entire retrograde process," removing the equivalent of more than 100 cargo planeloads of equipment from Afghanistan while turning over another 1,800 pieces of equipment to be destroyed.

But Central Command (CENTCOM) and the Pentagon declined to share information on how many of the 2,500 to 3,500 U.S. troops remain in Afghanistan, citing security concerns.

"We have an obligation to keep our people safe," Pentagon Press Secretary John Kirby told reporters Tuesday.

"We have to assume that this is going to be an opposed retrograde," he said. "If we assumed anything less it would be irresponsible of us."

The U.S. has sent elements of an Army Ranger task force to Afghanistan to help protect withdrawing U.S. and coalition forces. It has also sent six B-52 long-range bombers and 12 F-18 fighter-bombers to the region, and officials extended the deployment of the USS Dwight D Eisenhower carrier strike group to the North Arabian Sea to provide additional firepower, if necessary.

In the weeks leading up to the official start of the U.S. withdrawal, Taliban officials repeatedly threatened to target U.S. and coalition forces, arguing the foreign troops needed to be gone by May 1 - the deadline under an agreement signed between the Taliban and the administration of former U.S. President Donald Trump.

Despite those threats, though, Taliban commanders have focused their attacks on Afghan government forces.

On Monday, Kirby told Pentagon reporters that while the level of violence in Afghanistan was "still too high," U.S. commanders have not run into any problems that would slow down the U.S. pullout.

Following months of internal deliberations and consultation with allies, U.S. President Joe Biden announced last month that all U.S. forces would leave Afghanistan by September 11, the 20th anniversary of the terrorist attacks on New York's World Trade Center and the Pentagon, which were planned by the al-Qaida terror group in Afghanistan.

U.S. military and intelligence officials have voiced concern about the impact the withdrawal will have on their ability to collect information on terrorist groups and counter plots emanating from Afghanistan.

U.S. military officials have said they will rely on "over-the-horizon" surveillance and strike capabilities once all U.S. troops have left Afghanistan. But so far, officials say there has been little progress on security needed basing agreements with other countries in the region.

"There are very active discussions going on now inside the department to better define what over-the-horizon counterterrorism capabilities we'll be able to avail ourselves of," the Pentagon's Kirby said Tuesday.