South Sudanese troops and rebels battled Monday, defying mounting pressure to end four months of civil war, with US Secretary of State John Kerry warning of "serious" implications if fighting continues.

South Sudanese troops and rebels battled Monday, defying mounting pressure to end four months of civil war, with US Secretary of State John Kerry warning of "serious" implications if fighting continues.

A government offensive to seize the key northern oil town of Bentiu from rebel forces raged Monday, days after Kerry flew to Juba to extract promises from South Sudanese President Salva Kiir to hold direct talks with rebel chief Riek Machar on ending the killings in the world's youngest nation.

"Let me make clear: if there is a total refusal by one party or the other to engage into a legitimate promise which they agreed on... not only might sanctions be engaged but there are other serious implications and possible consequences," Kerry said, speaking in the Angolan capital Luanda on the last leg of his African tour.

So far, US-backed diplomatic efforts have struggled to gain traction, with both sides accused of war crimes including mass killings, rape, attacks on hospitals and places of worship and recruiting child soldiers.

A January ceasefire was never enforced, while stop-start peace talks in Ethiopia are yet to achieve even agreement on a basic agenda, despite UN warnings of the risk of famine and genocide if the fighting continues.

"There is accountability in the international community for atrocities, there are sanctions, there are possible... peacemaking forces, there are any number of possibilities," Kerry said.

Army spokesman Philip Aguer said there had been heavy fighting Monday in and around Bentiu, state capital of the oil-producing Unity state, a day after government troops moved to wrest back control.

"We are fighting in and around Bentiu to take back control," Aguer told AFP. "They are resisting but we have the upper hand."