Rebels in Yemen have detained five staff members of the International Medical Corps and two contracted drivers, the aid organisation and a local source said on Saturday.
"We are working to ensure this matter can be resolved as soon as possible," the Los Angeles-based relief agency said in a statement posted on Friday on its website.
The non-governmental organisation declined to reveal further details on Saturday, but a Yemeni aid official told AFP that the detained staff are all Yemenis, and were captured in the central province of Ibb.
The organisation's website says it has more than 150 local staff in Yemen and that it has operated since 2012 from three offices in the Arabian Peninsula country.
It says its relief efforts continue to provide a lifeline for families in the rebel-held capital and in Ibb, as well as the flashpoint southwestern city of Taez and Aden and Lahj in the south.
Shiite Huthi rebels and their allies control most of Yemen's north and west, while forces loyal to President Abedrabbo Mansour Hadi, backed by a Saudi-led Arab coalition, control the south and east.
Humanitarian agencies have long complained of the lack of access to people in dire need for help in war-torn Yemen.
The UN says seven million people face serious risk of famine unless international donors intervene, and has called for $2.1 billion in humanitarian aid.
More than 7,700 people have been killed since the coalition intervened in March 2015, most of them civilians, according to World Heath Organization figures.