Yemeni rebels and their allies charged Thursday that an American detained in the capital this week had provided target coordinates for air strikes by their foes in a Saudi-led coalition.
Masked gunmen wearing the uniform of the rebels' national security service seized Peter Willems on Tuesday from the principal's office of the Exceed Language Center he heads.
Students described scenes of panic as he was hauled off without any immediate explanation.
"Here is the American spy Peter Willems, director of the Exceed Language Center," a member of the rebels' Revolutionary Committee, Nayef al-Qanes, tweeted alongside a photograph of the detained school head.
"He was arrested in Sanaa after it was established that he was providing coordinates" to the coalition, Qanes added.
The same accusation was levelled by a close aide of ousted president Ali Abdullah Saleh, whose supporters in the army helped the rebels capture Sanaa in September 2014.
"He was providing information and coordinates to the coalition," tweeted Ibrahim Saryi, chief of staff of Saleh's powerful son Ahmed.
Willems is not the first American to be detained by the rebels.
In April, a US citizen was flown out of Sanaa to Muscat after successful negotiations for his release by Oman, the only Gulf Arab state which is not part of the Saudi-led coalition battling the rebels.
Last November, Oman evacuated three Americans who had been detained for spying by the rebels.
And in September last year, Oman helped to negotiate the release of a Briton, two Americans and three Saudis.
Hostility to Washington has long been a key part of the rhetoric of the Huthi Shiite rebels. They chant the same "Death to America" slogan at their rallies as used by the Shiite regime in Iran.
The hostility has increased since the Saudi-led coalition launched its military intervention in support of President Abedrabbo Mansour Hadi in March last year with reconnaissance and refuelling support from Washington.
Western embassies have long since quit the capital, with most diplomats now based in neighbouring countries.
Only a small number of Westerners remain in Sanaa.