The International Committee of the Red Cross (ICRC) said on Thursday it had pulled 71 international staff out of Yemen because of security incidents and threats, moving them to Djibouti.
The aid agency called on all the warring sides in Yemen's three-year conflict to provide security guarantees so it can keep running its surgical, water and food assistance programmes, which it said had been crippled by the partial evacuation.
Some 450 ICRC employees remain in Yemen, including dozens of expatriate staff, spokeswoman Marie-Claire Feghali said.
"Our current activities have been blocked, threatened and directly targeted in recent weeks, and we see a vigorous attempt to instrumentalize our organization as a pawn in the conflict," the ICRC said in a statement.
An ICRC employee, a Lebanese national, was killed on 21 April by an unknown gunman who opened fire on his car in the southwestern Yemeni city of Taiz as he was on his way to visit a prison, it said at the time.
"While the Yemen delegation has received numerous threats in the past, we cannot now accept additional risk less than two months after a gunman killed a staff member. The security of our staff, who are being intimidated by parties to the conflict, is a non-negotiable prerequisite for our presence and work in Yemen and an absolute priority," the statement said.
Saudi Arabia, the United Arab Emirates and Sunni Muslim allies entered the war in 2015 to try to drive back the Houthis, an Iran-allied Shi'ite movement that forced a Saudi-backed government into exile in 2014
It was not immediately clear whether any UN or other aid agencies would begin evacuating their foreign staff from Yemen. Unidentified forces attacked a UN aid vessel off the main Yemeni port of Hodeidah at the weekend, port authorities have said.
As forces of the Saudi-led military coalition close in on Hodeidah, aid agencies fear a major battle will shut down a vital lifeline for millions of hungry civilians.