WASHINGTON — Four American military personnel assigned to the United States Embassy in Tripoli, Libya, were detained Friday and were being held by the country's Interior Ministry, American officials said.

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According to initial reports received by officials in Washington, the four were believed to have been reviewing potential evacuation routes for diplomats when they were detained. The circumstances of how they were taken and why were not clear.

Photographs of two American passports and embassy identity cards were later disseminated on Twitter. The State Department had no immediate comment.

The episode appears to have taken place in a town just southwest of the historic Roman ruins at Sabratha and about an hour's drive from Tripoli, the capital. The area is not known for anti-Western extremists or other obvious threats. In part because it is a tourist area, the district around Sabratha skews relatively liberal and friendly to Westerners.

Since the attack on the United States Mission in Benghazi that killed Ambassador J. Christopher Stevens on Sept. 11, 2012, employees of the American Embassy have operated with extraordinary caution. Rigorous security rules preclude any movements outside the heavily fortified embassy compound without advance planning and an armed guard. The compound is locked at night, and no one is permitted to enter or exit. Counterterrorism has become a central focus of the work there, and the compound brims with well-armed security officers.

But two years after the toppling of Col. Muammar el-Qaddafi, security remains tenuous even in and around Tripoli. Libya's transitional government has not yet managed to assemble a credible national army or police force. Many families or clans around the country keep heavy weapons, as do autonomous local militias formed during and after the Libyan uprising.