(Reuters) - Evidence collected by Human Rights Watch show that widely banned cluster bombs have been used in Libya's conflict between two rival governments fighting for control of the North African country, the New York-based group said on Sunday.

Cluster bombs explode in the air, scattering dozens of smaller bomblets over an area the size of a sports field. Most nations have banned their use under a convention that became international law in 2010 but Libya has not signed it.

Reuters journalists were shown what appeared to be cluster bomb remnants by forces loyal to a self-declared government controlling Tripoli at two sites in central Libya which had been bombed by war planes belonging to the official government.

Libya is racked by conflict between rival governments -- the internationally recognised one based in the east and a rival outlet controlling Tripoli -- fighting for territory and oil facilities four years after the fall of Muammar Gaddafi.

"Phone interviews with witnesses and photographic evidence reviewed by Human Rights Watch indicate that remnants of RBK-250 PTAB 2.5M cluster bombs were found at Bin Jawad in February 2015 and at Sirte in March," HRW said in a report.

A Reuters photographer took pictures of what looked like cluster bomb remnants in the central city of Sirte while another Reuters reporter saw such bombs in Ben Jawad, where forces loyal to the rival government are based.

"The good condition of the paint on the bomb casings and lack of extensive weathering indicated that the remnants had not been exposed to the environment for long and were from a recent attack," HRW said.

It said it was not possible to say who was responsible for the use of cluster bombs but both locations have been repeatedly bombed by planes belonging to the internationally-recognized government.

Its air force commander, Saqer al-Joroushi, denied that his side had used such bombs.

"The new evidence of cluster munitions use in Libya is highly disturbing," said Steve Goose, arms director at Human Rights Watch and chair of the Cluster Munitions Coalition, in the report. "Libyan authorities should investigate these incidents and make sure its forces don't use cluster munitions."

Both side in the conflict have been accused by rights groups of attacking civilian facilities such as densely-populated residential districts, oil ports, airports or commercial harbours.

(Reporting by Ulf Laessing; Editing by Mark Heinrich)