UNITED NATIONS (Reuters) -Five United Nations security staff who were kidnapped in Yemen by al Qaeda militants 18 months ago have been released, the United Nations said on Friday.

U.N. Secretary-General Antonio Guterres welcomed the release and "reiterates that kidnapping is an inhumane and unjustifiable crime, and calls for the perpetrators to be held accountable," a U.N. spokesperson said.

The staff - four from Yemen and one from Bangladesh - were in "very good health and good spirits, despite everything that they went through," the top U.N. official in Yemen, David Gressly, told reporters.

"But they went through a very difficult period of 18 months of isolation," he added.

The U.N. staff were kidnapped in Yemen's southern region of Abyan on Feb. 11, 2022. Yemen-based Al Qaeda in the Arabian Peninsula (AQAP) has used a conflict between a Saudi-led coalition and the Iran-aligned Houthis to enhance its influence.

Gressly warned that AQAP was an "increasing threat."

Yemen has been mired in conflict since the Houthi group ousted the government from the capital Sanaa in late 2014. The Saudi Arabia-led military coalition intervened in 2015, aiming to restore the government.

Peace initiatives have seen increased momentum since Riyadh and Tehran in March agreed to restore diplomatic ties severed in 2016.

(Reporting by Michelle Nichols; editing by Rami Ayyub and Jonathan Oatis)