WASHINGTON (Reuters) - The United States is concerned about worsening security in war-torn Central African Republic and urged all sides to implement January's ceasefire deal, the State Department said on Sunday.

The rebel Seleka coalition and President Francois Bozize's government signed a ceasefire agreement in Libreville, Gabon, in January to end an insurgency that swept to within striking distance of the capital Bangui.

The deal also included forming a government of national unity, but the rebels did not take up key posts in that government in February, waiting for Bozize to free political prisoners and the withdrawal of most foreign forces brought in to shore up the army during the uprising.

Early this month, Seleka, a grouping of five rebel movements, seized the key south-eastern town of Bangassou.

"We call on President (Francois) Bozize and the leadership of the Seleka alliance to cease hostilities immediately, and implement the provisions of the Libreville Agreement," State Department spokeswoman Victoria Nuland said in a statement.

"We are very concerned by the worsening humanitarian situation in CAR and credible, widespread reports of human rights abuses by both national security forces and Seleka fighters."

Nuland said the United States urged the Economic Community of Central African States to urgently convene a mediation committee, in line with the Libreville Agreement, to support the national unity government and help restore peace and security.

"The United States urgently calls on the Seleka leadership and on the CAR government to ensure that their forces respect the human rights of the Central African people. Perpetrators of such abuses must be held accountable."

Central African Republic is one of a number of countries in the region where U.S. Special Forces are helping local soldiers hunt down the Lord's Resistance Army, an unrelated rebel group that has killed thousands of civilians across four nations.

The country remains one of the least developed on the planet despite rich deposits of gold, diamonds and uranium.

(Reporting by Lucia Mutikani; Editing by Philip Barbara)