U.N. officials say the nation, one of the world's poorest with a long history of chaos and coups, has been collapsing since rebel groups joined forces to overthrow the president in March and put their leader in charge.
Human rights groups have accused rebel forces of committing scores of atrocities, widespread looting and conscription of child soldiers.
Farrow said a larger U.N.-authorized force with a stronger mandate is needed.
Last month, the U.N. Security Council unanimously backed a new African Union peacekeeping force, which was established in the country in July and has about 1,900 of its authorized 3,600 troops.
"When you see a failed state in that region it's a calling card to extremist groups," Farrow said.
Farrow previously visited the country representing the U.N. children's agency in 2007 and 2008.