The US has sent 50 troops to Chad to help evacuate US citizens and embassy staff in neighbouring Central African Republic where rebels have seized several cities and are advancing on the capital Bangui.
Barack Obama, the US president, informed congressional leaders of Thursday's deployment in a letter on Saturday citing a "deteriorating security situation" in the deeply impoverished nation.
The US has special forces in the country who are assisting in the hunt for Joseph Kony, a Ugandan fugitive rebel leader of the Lord's Resistance Army based in northern Uganda.
The planned evacuation of the US diplomats follows criticism of Washington's handling of diplomatic security before and during the attack on its consulate in Benghazi, Libya, on September 11.
The ambassador and three other Americans were killed in that attack.
The US deployment comes amid reports that a rebel coalition known as Seleka seized another town in their advance on the capital, forcing an army retreat and putting the rebels just one town away from Bangui.
Sibut, a key transport hub, fell on Saturday without a shot being fired because the government army and forces from Chad had pulled back to Damara, 75km from Bangui on Friday, said Josie Binoua, minister of territorial administration.
The report could not be independently verified as telephone lines to Sibut have been cut, making it difficult to check the situation with local residents.
Talks between the rebels and the government led by President Francois Bozize are planned to start next week in Gabon.
Representatives from the 10-nation Economic Community of Central African States agreed at a meeting in Gabon on Friday to send forces to CAR. However, they did not did not specify how many troops would be sent or how quickly the military assistance would arrive.
As fears mounted that the rebels would attack Bangui, which has 600,000 residents, Bozize pleaded for international help. But former colonial power France said its forces in the country are there to protect French interests and not Bozize's government.
The United Nations Security Council has condemned the violence and reiterated its demand that the armed groups "immediately cease hostilities, withdraw from captured cities and cease any further advance towards the city of Bangui".
The UN also announced that it was pulling its non-essential staff out of the country.
The landlocked nation of 4.4 million people has suffered decades of army revolts, coups and rebellions since gaining independence in 1960 and remains one of the poorest countries in the world.
Bozize's government earlier reached out to longtime ally Chad, which pledged to send 2,000 troops to bolster its own forces.
The rebels behind the most recent instability signed a 2007 peace accord allowing them to join the regular army, but insurgent leaders say the deal was not fully implemented.
The rebels say they are fighting because of their "thirst for justice, for peace, for security and for economic development of the people of Central African Republic".