The US Navy has stationed the Wasp-class amphibious assault vessel USS Iwo Jima and the Whidbey Island-class dock landing ship USS Fort McHenry on standby for the potential evacuation of US embassy personnel in Yemen, amid a worsening security crisis in the country.

The warships are on regular deployment in the Red Sea in the US Central Command area of responsibility.

Pentagon spokesperson army colonel Steve Warren said: "We are continuing to closely monitor the situation in Yemen.

"We are continuing to closely monitor the situation in Yemen."

"The [USS] Iwo Jima and [USS] Fort McHenry are on station, and between those two warships there is enough combat power to respond to whatever contingency may come up."

The navy’s 24th Marine Expeditionary Unit in the Gulf of Aden off the coast of Yemen has not yet made any additional moves in response to rising violence.

Meanwhile, the US State Department has so far not issued orders to close the embassy, which has been functioning with reduced staffing and increased security since 24 September 2014.

An unidentified US official with direct knowledge of the planning was quoted by CNN as saying that if an evacuation is ordered the operation could last for several days.

"Nobody should think this would be easy," the official said.

According to state news agency SABA, Houthi militiamen, a group representing the country’s Zaidi sect, which is an offshoot of Shiite Islam, took over the presidential palace.

In addition, a US diplomatic vehicle was attacked by militants at a checkpoint near the embassy, although no injuries were reported.

The number of US citizens in Yemen remains undisclosed.