Secretary general of the Union of Popular Forces party Mohammed Abdulmalik al-Mutawakil was shot dead on Sunday by a gunman on a motorbike on a street in central Sanaa.
“We urge the Yemeni government to conduct a full investigation that brings to justice those responsible for this crime,” State Department spokeswoman Jen Psaki said, offering condolences to his family.
“Yemen’s challenges, particularly now, require moderate voices and a commitment to the rapid and peaceful resolution of differences,” she added.
It was not immediately clear why Mutawakil was targeted.
The attack came a day after Shiite rebels and their political rivals, including Mutawakil’s party, signed an agreement on the formation of a new technocrat government to resolve the country’s crisis.
Psaki highlighted that the UN was mulling sanctions against those people “whose actions risk destabilizing Yemen or threaten the transition process.”