The United States told its citizens in Yemen to leave and said it was reducing the number of U.S. government staff there due to political unrest and fears of a possible military escalation.

President Abd-Rabbu Mansour Hadi has said Yemen may be heading for civil war after Houthi rebels overran the capital Sanaa, allowing them to dictate terms to his weakened, fractured government. Over 100 people have been killed in fighting.

The Department of State warned "U.S. citizens of the high security threat level in Yemen due to terrorist activities and civil unrest," it said in a travel warning.

On Wednesday, the State Department ordered a reduction in the number of U.S. government personnel in Yemen "out of an abundance of caution due to ... the potential for military escalation". The U.S. Embassy would continue with its operations, it said.

Washington announced a temporary suspension of operations at its embassy in May after attacks on foreigners by al Qaeda.

Any possible civil war would likely pit the Houthi rebels against an alliance of Sunni Islamist and tribal interests united by a few top families and generals. Houthis say they represent Zaydi Shi'ite Muslims, about 30 percent of Yemen's 25 million people.

The Houthis struck a deal that will make them a part of the government, but it is not clear if that will satisfy their demands, or if it will instead embolden them to seek further powers.

Any renewed fighting could allow an array of other interests, including southern separatists, former president Ali Abdullah Saleh and even al Qaeda to try to take advantage.

An aide to Barack Obama on counterterrorism, Lisa Monaco, phoned President Hadi on Wednesday to express the U.S. president's support and his condemnation of the Houthis.

Yemen is located on major oil shipping routes and next to top Gulf crude exporters. Washington is keen to prevent any spillover of violence into neighboring oil power Saudi Arabia and to stop Yemen being used as a springboard to attack Western targets.