The European Union has hit out at the decision by the United States to label Yemen's Huthi rebels as "terrorists", warning it could hinder peace efforts and aid deliveries.
Outgoing President Donald Trump's administration announced the last-ditch move on Sunday.
The designation is set to come into force on the eve of the inauguration of his successor Joe Biden next week, whose aides had hoped to mount a fresh push to end Yemen's six-year war.
It has been seen as complicating the incoming US leader's promised efforts to restart diplomacy with Iran, which has links to the Huthis.
The decision has drawn criticism from aid groups and the United Nations over fears it will exacerbate the already dire humanitarian crisis in war-ravaged Yemen and could tip the country in famine.
A spokesman for EU foreign policy chief Josep Borrell said the US decision "risks rendering UN-led efforts to reach a comprehensive solution to the Yemen conflict more difficult".
"It will complicate the necessary diplomatic engagement with Ansar Allah and the work of the international community on political, humanitarian and developmental matters," a statement said, using the official name of the Huthi movement.
"The designation is likely to have disruptive effects on the delivery of humanitarian aid funded by the international community and further aggravate the economic crisis which has resulted from over five years of conflict."
The rebels control much of Yemen and have faced a bloody offensive from US-ally Saudi Arabia, with millions in the country depending on relief to survive.
A designation as a terrorist group is expected to scare away outside actors from many transactions with Huthi authorities, including bank transfers and buying food and fuel, for fear of US prosecution.
Aid groups have warned against the blacklisting of the Huthis, saying that they have no option but to deal with what is the de facto government in northern Yemen.