WASHINGTON (Reuters) - The State Department said on Tuesday three U.S. citizens are in Yemeni custody on terrorism-related charges but none was arrested recently.

The Saudi-owned al-Hayat newspaper on Monday reported that Yemen has detained around 50 foreigners accused of links to al Qaeda after stepping up monitoring of Arabic language schools.

Responding to questions triggered by the report, State Department spokesman P.J. Crowley on Monday said there were 12 U.S. citizens in custody in Yemen but gave no details on why they were detained.

On Tuesday, Crowley said that he did not know of any recent arrests of U.S. citizens in Yemen. He also said that the number of Americans in custody in the impoverished Arab nation varies and can be as high as about 20 at any given time.

"We are not aware of any new arrests on terrorism charges at this point," Crowley said on Tuesday.

"Right now, we are only aware of three American citizens in custody in Yemen on terrorism charges," he added, saying that the three had been arrested since the beginning of the year.

Al-Hayat said U.S., British, French and Malaysian nationals were among the foreigners detained since a failed December, 2009 attempt to bomb a U.S.-bound plane. The Nigerian suspect in that case had studied Arabic in the Yemen's capital, Sanaa.

Yemen's Western allies and Saudi Arabia fear al Qaeda is trying to exploit instability in Yemen to use the impoverished country, with domestic conflicts in its north and south, as a base to launch attacks in the region and beyond.

(Editing by Paul Simao)