The US embassy was already working on bare bones staffing after most of the diplomatic personnel was ordered to leave in September at the start of a Shiite militia sweep into the capital, Sanaa.
"In response to the changing security situation in Yemen, the United States Embassy in Sanaa has further reduced its American personnel working in Yemen," a senior State Department official said.
The embassy remained opened though, and "we will continue to operate as normal, albeit with reduced staff," the official added yesterday.
Yesterday's events appeared to catch Washington by surprise, with the State Department saying it was "seeking confirmation" of what was happening.
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"We continue to support a peaceful transition. We've urged all parties and continue to urge all parties to abide by ... the peace and national partnership agreement," spokeswoman Jen Psaki told reporters earlier.
President Abdrabuh Mansur Hadi resigned yesterday saying the country was in "total deadlock" after the powerful Huthi Shiite militia took control of most of Sanaa.
Hadi's government has been a key ally of the United States, allowing Washington to carry out repeated drone attacks on Al-Qaeda militants in its territory.
It was not immediately clear how the staff had been withdrawn, but two amphibious ships with US Marines on board are currently off the Yemeni coast in case any evacuation is ordered.
The United States was hoping that dialogue will continue on the ground between the Yemeni factions as "that's the only way, in our view, to de-escalate the situation on the ground," Psaki said.