Khartoum, Oct 27 (IANS) At least 20 soldiers were killed in clashes with armed herdsmen in South Sudan's state of Buhairat Friday, army said Sunday.

"A group of armed herdsmen Friday attacked a force belonging to the army at an area some 40 miles from Rumbaik, the capital city of Buhairat state," army spokesman Philip Aguer told Xinhua over phone Sunday.

"We do not know the reason, but what we hear from our field officers is that over 20 soldiers, including an officer, were killed," he said.

The spokesman said that South Sudan is witnessing relevant calm in recent days, but he still acknowledged limited clashes in western Upper Nile state. "The rebel forces, led by Riek Machar, are attacking areas at western Upper Nile state and South Sudan army is responding in self-defence."

South Sudan has been witnessing violent armed clashes between the government army and defectors loyal to former vice-president Riek Machar since mid-December last year.

The clashes have left thousands of South Sudanese dead, forcing around 1.8 million people to flee homes in the world's youngest nation.