The Washington Post reported that the Americans were believed to be held by the Houthi militia in a prison near the capital Sanaa, and that U.S. efforts to secure their release had faltered, hampered by the fact that Washington has no direct links to the rebels.

The State Department had seen the “reports that several U.S. citizens have recently been detained in Yemen,” the official told AFP.

“We are doing everything we can to get these individuals released,” the official said, but would not say how many were held or who was holding them.

“The protection of U.S. citizens abroad is a top priority,” he added, saying for privacy and safety reasons no further information could be given.

US officials, quoted by the Post, said none of those held were U.S. government employees.

Three of the prisoners were believed to work in the private sector, while a fourth was a dual U.S.-Yemen citizen.

Saudi Arabia, aided by US intelligence, has been leading an air war since March 26 targeting the Iran-backed Houthi Shiite rebels.

The militia has seized parts of the country and forced the Western-backed President Abdrabbo Mansour al-Hadi to flee to Riyadh earlier this year, in a conflict which has claimed an estimated 2,000 lives.

Clashes have also raged on the ground between the rebels and local militia fighting their advance, especially in southern Yemen.

Amid the violence, Washington closed its embassy in Sanaa in February and has temporarily relocated its ambassador and staff in Jeddah, Saudi Arabia.

US officials said that efforts to secure the release of the Americans had been mainly through “intermediaries including humanitarian groups that continue to have a presence in Sanaa,” the Post said.

One of the prisoners had been approved to be released in recent days, but the rebels went back on their decision, the Post said.

He had initially been detained for overstaying his visa, but then the rebels accused him of travelling to “sensitive” areas in Yemen, a U.S. official told the Post.

A fifth American, identified by the daily as Sharif Mobley, is being held on terrorism charges brought against him more then five years ago, the paper said.