The United Nations said on Saturday two contractors with the world body had been detained in Yemen, but gave no further details on the incident.
"Two contractors have been detained and the Deputy Secretary-General (Jan Eliasson) is looking into it," a UN spokesman said without elaborating or confirming if the two were American citizens.
A US State Department official said the department was aware of reports that two US citizens had been detained at Sanaa airport in Yemen.
"The protection of US citizens overseas is one of the Department of State's highest priorities, including in these cases," the official said in an email. "Due to privacy considerations, we do not have any further information to share."
A Saudi-led Arab coalition intervened in Yemen's civil war in March to try to restore the government after it was toppled by Iran-allied Houthi forces, but a mounting civilian death toll and dire humanitarian situation has alarmed human rights groups.
The United Nations has designated Yemen as one of its highest-level humanitarian crises, alongside emergencies in South Sudan, Syria and Iraq. It says more than 21 million people in Yemen need help, or about 80 percent of the population.