Government troops have captured a rebel stronghold and taken back control of another town, sending rebels fleeing toward the Ethiopian border, a South Sudanese military spokesman said on Monday.

The government offensive comes just days after South Sudanese President Salva Kiir told US Secretary of State John Kerry that he was ready to hold peace talks with the rebel leader, former vice-president Riek Machar, after a peace deal signed in January had failed to hold.

Government troops have taken over the rebel base of Nasir, in the Upper Nile state, and re-captured the capital of the oil-producing Unity state, Bentiu, from rebel control, Colonel Philip Aguer said. Bentiu was taken after a day-long exchange of fire on Sunday with an unknown number of casualties, he said. Nasir was the rebel headquarters from where the rebels were mobilizing to attack the town of Malakal, he said. He said Machar and his troops are now somewhere near the Ethiopian border.

Kerry met with President Kiir on Friday in South Sudan's capital Juba and afterward announced during a news conference that Kiir had expressed willingness to meet with Machar. Kiir did not address the media. He flew to the Kenyan capital of Nairobi where he met with Kenyan President Uhuru Kenyatta and Ugandan President Yoweri Museveni and announced that he is willing to hold the talks with Machar to discuss ways and means of ending the conflict in the country.

Kerry was in South Sudan to urge its leaders and rebels to end months of ethnic killing that have raised the specter of genocide in the world's newest nation.

South Sudan has been rocked by violence since December, when Kiir accused Machar of staging a coup. An unknown number of people have died and 1 million people have fled their homes due to the conflict. With few residents tending crops, UN officials say the country faces a severe risk of famine in the months ahead.

Raging violence

The violence is increasingly taking on an ethnic dimension between Kiir's Dinka community and Machar's Nuer community.

Rebel fighters from the Nuer ethnic group took control of Bentiu mid-April and slaughtered non-Nuer civilians in the town mosque, the hospital and on streets, leaving "piles and piles" of bodies, the UN's top aid official Toby Lanzer said. The UN Security Council expressed "horror" at the massacre. It said council members may be willing to impose sanctions if attacks on civilians continue.

Kiir fired the country's top military officer last month, Chief of Staff James Hoth Mai, further isolating the Nuer group politically. Mai is Nuer and his command position, which he held since 2009, was frequently cited as an example of the ethnic diversity of the government led by Kiir, an ethnic Dinka.

Machar has said he wants to see the exit of Kiir, whom he accuses of acting like a dictator. Kiir accuses Machar of launching a failed coup attempt in December 2013 that the government says sparked unrest across the country.

AP-AFP