The United Nations said Monday it suspended its humanitarian operations in the stronghold of Yemen’s Houthi rebels after they detained eight more U.N. staffers, affecting the global response to one of the world’s worst humanitarian disasters.

In a statement, the U.N. said the ‘extraordinary’ decision to pause all operations and programs in northern Saada province was due to the lack of necessary security conditions and guarantees.

A spokesman for the Houthis didn’t immediately respond to messages seeking comment.

The rebels in recent months have detained dozens of U.N. staffers, as well as people associated with aid groups, civil society and the once-open U.S. Embassy in Sanaa, Yemen’s capital. None of the U.N. staffers has been released.

The U.N. statement said the pause in operations is meant to give the Houthis and the world body time to ‘arrange the release of arbitrarily detained U.N. personnel and ensure that the necessary conditions are in place to deliver critical humanitarian support’ in rebel-held areas.

It said the latest detained U.N. staffers — taken late last month — included six working in Saada, on Yemen’s northern border with Saudi Arabia.

Seven U.N. agencies operate in Saada, including the World Food Program, the World Health Organization and UNICEF, along with several international aid organizations, according to the U.N. humanitarian agency.

The U.N. late last month suspended all travel into Houthi-held areas.

The war in Yemen has killed more than 150,000 people, including fighters and civilians. The Iranian-backed Houthis have been fighting Yemen’s internationally recognized government, which is backed by a Saudi-led coalition, since 2014, when they descended from their stronghold in Saada and took control of Sanaa and most of the north.

The U.N. had projected that over 19 million people across Yemen will need humanitarian assistance this year as many deal with climate shocks, malnutrition, cholera and the economic effects of war.

The rebels have imprisoned thousands of people during the war. In recent months, they also intensified their crackdown on dissent, including recently sentencing 44 people to death.

In January, the Houthis unilaterally freed 153 war detainees as one of several overtures to ease tensions after the ceasefire in the Israel-Hamas war in Gaza. Such prisoner releases have been viewed as a means to jump-start talks over permanently ending Yemen’s war.