The military personnel will protect United Nations political mission in the country.
About 250 military personnel were deployed to Bangui in the first phase of the programme.
In a letter to the 15-member Security Council, UN Secretary-General Ban Ki-moon said that the guard unit would, in a second phase, increase its strength to a battalion size unit of 560 military personnel, with its own enablers, in order to progressively deploy to locations outside Bangui where the United Nations has a presence.
The mineral-rich country slipped into chaos since northern Seleka rebels seized the capital, Bangui, and ousted President Francois Bozize in March.
UN officials and rights groups say both sides may have committed war crimes.
The UN Security Council adopted a resolution this month urging the United Nations to consider establishing a full-fledged peacekeeping force and asking Ban for interim plans for a guard force to protect the UN Integrated Peacebuilding Office, known as BINUCA.
“Given the urgency of the situation, as an interim measure the 250 troops could be temporarily redeployed from another UN peacekeeping operation.”
The guards would provide perimeter security and access control.
The UN Security Council is expected to approve the UN guard force on Friday, diplomats said.
France, which intervened this year to oust Islamist rebels from another of its former colonies, Mali, has been reluctant to get directly involved. It has urged African nations and the African Union to do their utmost to resolve the crisis among themselves.
But while the African Union plans to deploy a 3,600-member peacekeeping mission in the country – known as MISCA – incorporating a regional force of 1,100 soldiers already on the ground, it is unlikely to be operational before 2014.
Some Western diplomats say the situation in Central African Republic is too fragile to permit the deployment of a UN peacekeeping force in the foreseeable future.
France has a small force in Bangui securing the airport and its local interests. French diplomatic sources have said Paris would be ready to provide logistical support and increase its troop numbers to between 700 and 750 if needed.
Central African Republic is rich in gold, diamonds and uranium.