U.S. military officials on Friday disclosed that four air strikes inside Yemen have killed 15 al-Qaida militants dating back to February and March.

Yemen is one of the terror group's primary strongholds in the region. The newly announced strikes bring to nine the total there so far in 2016, and highlight an intensifying U.S. mission that also includes a small team of American ground troops.

Those forces are operating near the Yemeni port of al Mukalla, a safe haven for the group known as al-Qaida in the Arabian Peninsula, or AQAP.

The air strikes, announced by U.S. Central Command, include:

In March 2015, the U.S. evacuated about 125 special operations troops amid the expanding civil war between Yemen's government loyalists backed by a Sunni Arab coalition and Houthi rebels supported by Iran. Shortly after that withdrawal, al-Qaida militants seized territory along Yemen’s coastline, including al Mukalla, and used lucrative oil exports to help transform the city into a wealthy ministate.

The small team of U.S. troops deployed in April to a “fixed location” in Yemen to provide support for operations led by the Yemeni military and the United Arab Emirates. Pentagon officials said those troops are providing intelligence support as well as airborne intelligence, surveillance and reconnaissance, advice and assistance with operational planning, maritime interdiction and security operations, medical support and aerial refueling.

The deployment of those U.S. troops is "temporary," officials said, and both operationally and legally distinct from another U.S. military operation in Yemen, one that is providing support for a Saudi-led coalition backing Yemeni government troops. Under that arrangement, the U.S. is offering the Saudis intelligence, airborne fuel tankers and thousands of advanced munitions in the fight against Iranian-backed rebels.

A CENTCOM spokesman, Air Force Col. Pat Ryder, offered no specific reason for the unusual delay in announcing the air strikes in Yemen.

"As part of Central Command's commitment to transparency, we will endeavor to release information about counterrerrorism strikes in places like Yemen, recognizing that there may be times when such announcements may be delayed for operational or intelligence reasons," Ryder said Friday.

"Sometimes there is going to be competing priorities in terms of operations and intelligence, collecting reflections on strikes, not wanting to let our adversaries know where those strikes are coming from. ... Sometimes the chatter that comes after a strike enables us to collect more intelligence on adversaries and conduct future strikes."

Al-Qaida in the Arabian Peninsula "remains a significant threat to the region and has a destabilizing effect on Yemen," Ryder added. "The terrorist group is using the unrest in Yemen to provide a haven from which to plan future attacks against our allies as well as the U.S. and its interests."