South Sudan: Heavy Fighting Grips Key Oil Town of Bentiu

South Sudan's military has launched a major assault against rebels holding the key oil-producing hub Bentiu. The US top diplomat John Kerry has warned both sides to begin peace talks soon or face "serious" consequences.

The town of Bentiu was the site of heavy fighting on Monday, according to South Sudanese officials. Last month, the oil-producing center, which is also the capital of Unity state, came under the control of rebel forces who oppose the government under President Salva Kiir.

"We are fighting in and around Bentiu to take back control," South Sudanese Colonel Philip Aguer said. "[The rebels] are resisting but we have the upper hand."

Meanwhile, government forces had retaken the city of Nasir, which lies less than 50 kilometers (31 miles) from the border with Ethiopia. According to South Sudan, rebel troops under the leadership of Riek Machar were retreating toward the eastern neighboring country.

"All is quiet [in Nasir], with the forces of Machar on the run," Colonel Auger said, adding that South Sudanese officials believed the rebel leader to be very close to the Ethiopian border.

Machar's spokesperson Yohanis Musa Pouk confirmed that the chief of the government opposition forces remained in the country, in an interview with the Associated Press news agency.

Fighting broke out in South Sudan last December after President Kiir alleged that Riek Machar - his former vice president - had attempted to stage a coup. Thousands have been killed since in the violent unrest and at least one million people have been displaced.

Stern warning from US' John Kerry

United States Secretary of State John Kerry - who visited South Sudan late last week - reacted strongly to the situation in South Sudan on Monday, criticizing the fact that neither side appeared to be working toward making peace plans come to fruition.

"Let me make it clear, if there is a total refusal by one party or the other to engage... not only might sanctions be engaged, but there are other serious implications and possible consequences," Kerry said while speaking from Angola, the latest stop on his Africa visit.

Both sides have accused each other of committing a number of war crimes, including rape, attacks on places of safe haven, recruiting child soldiers and mass killings.

"The parties need to recognize that they signed a cessation of hostilities agreements, both of them, and the international community is prepared to take steps to see that it is honored, by putting additional troops," Kerry added.

Amid fears of genocide and famine, the United Nations has warned that South Sudan faces a catastrophe if efforts aren't made soon to end the war and has called for more humanitarian assistance.