South Sudan rebels and government troops battled Monday over the strategic town of Nasir, the United Nations said, with rebels launching their largest offensive since an oft-broken May truce.

Heavy shooting continued for a second day with fighting continuing in the northern town and rebel forces apparently in "firm control" of the centre, UN spokesman Joe Contreras said.

The United Nations warned Sunday that the fresh rebel offensive "represents the most serious resumption of hostilities" since President Salva Kiir and his former deputy, rebel leader Riek Machar, met in May promising again to stick to a January ceasefire.

The United States condemned the rebel attack on Nassir saying the town's residents "have suffered from frequent and horrific acts of violence and human rights abuses since fighting broke out in mid-December, causing widespread displacement and a worsening humanitarian crisis as civilians fear returning to their homes."

"We call on both parties to immediately end all such attacks and fully adhere to their ... commitments to cease hostilities," deputy State Department spokeswoman Marie Harf said in a statement.

She warned that "famine conditions" were looming in some areas of the country, making it "increasingly urgent that both parties immediately recommit themselves to inclusive, political negotiations", recalling that leaders from both sides were on notice of possible US sanctions for any human rights abuses or for threatening peace.

AU and IGAD condemnation

The African Union Commission (AUC) and the Inter-Governmental Authority on Development (Igad) have also strongly condemned the attack by the rebels. Igad has been mediating the South Sudan peace between Dr Machar's group and the government led by President Salva Kiir.

"The attack is in violation of the recommitment made by the parties, in Addis Ababa, on 9 May 2014, to fully comply with the Cessation of Hostilities Agreement," said AUC chairperson Nkosazana Dlamini-Zuma.

Dr Dlamini-Zuma called on all the South Sudanese parties, in particular the SPLM/A (in opposition), to fully comply with all agreements they had signed and immediately return to the negotiating table.

She also called on them to respect the integrity of the Monitoring and Verification Mechanism (MVM), as well as to guarantee the safety of its monitors, who have been deployed in various locations in South Sudan, including in Nassir town.

Igad said: "This has also happened after 10th June, 2014 Igad Summit, in which the two principals pledged to "end the war now" and establish a transitional government within 60 days. While the casualties of this attack are yet to be assessed, the mediation team is saddened by the continued loss of lives, not only of combatants, but of vulnerable groups like women and children."

Looming famine

More than seven months of war has left thousands dead and displaced 1.5 million people, and aid agencies are warning of famine if fighting continues. UNMISS, the UN mission, laid the blame for the truce violation squarely with Machar's forces.

READ: South Sudanese children starving while aid falling short

The rebels claim to control the town, their former headquarters, located some 500 kilometres (300 miles) north of Juba and close to the Ethiopian border.

But the United Nations said that fighting was going on, with the heaviest clashes Monday reported around the government army barracks, just west of the town. UN peacekeepers remain in control of their base, where more than two dozen civilians are sheltering inside.

Fighting in South Sudan had eased since May, in part due to heavy rains that have hampered troop movements. Previous ceasefire deals have failed to stick, and peace talks in luxury hotels in the Ethiopian capital Addis Ababa have made little progress.

Last month they halted indefinitely, with both sides refusing to attend the discussions, and blaming each other for the failure.

Earlier this month the departing UN representative in South Sudan Hilde Johnson issued a scathing attack on the country's leaders, lashing out at both the government and rebels, calling them a "self-serving elite" responsible for a looming "man-made famine".

READ: Leaders, rebels to blame for famine

Civilians have been massacred and dumped in mass graves, patients murdered in hospitals and churches, and entire towns flattened as urban centres, including key oil-producing hubs, changed hands several times.