South Sudan's government troops and rebels have engaged in heavy fighting, violating a ceasefire signed in May.

The clashes broke out on Friday and continued for several hours near the town of Bentiu, the capital of the oil producing Unity state.

The two sides accused each other of launching the attacks and violating the truce.

"It is a continuous violation of the ceasefire agreement," army spokesman Joseph Marier Samuel said.

However, rebel spokesman Lul Ruai Koang claimed that it was the beginning of a "long-awaited government offensive."

South Sudan plunged into violence in December 2013, when fighting erupted between troops loyal to President Salva Kiir and defectors led by his former deputy Riek Machar around the capital Juba.

The conflict soon turned into an all-out war between the army and defectors, with the violence taking on an ethnic dimension that pitted the president’s Dinka tribe against Machar’s Nuer ethnic group.

Under the Cessation of Hostilities Treaty signed on May 9, Kiir and Machar had agreed to establish a unity government by August 10, but they missed the deadline.

Last Friday, the United Nations Security Council also threatened to impose sanctions on both sides for their failure to resolve the ongoing conflict in the country.

The clashes have left thousands of South Sudanese dead and forced around 1.5 million people to flee their homes in the world’s youngest nation.

The widespread displacement has contributed to mass hunger and food shortages. Aid workers warn of famine if fighting continues.

South Sudan gained independence in July 2011 after its people overwhelmingly voted in a referendum for a split from the North.