South Sudan's civil war entered its ninth month Friday with rebels and government troops engaged in heavy battles, aid workers said, days after UN warnings of sanctions if the conflict continues.

Fighting was reported around the town of Bentiu, capital of the northern oil state of Unity, as well as in Jonglei state.

Rebel spokesman Lul Ruai Koang claimed it was the start of a "long awaited government offensive," reporting clashes to the east and south of Bentiu.

He also said there was fighting in the remote village of Ayod in the eastern Jonglei state, one of the areas hardest hit by a growing hunger crisis.

Aid workers also reported heavy shooting around Bentiu, which has swapped hands between rebels and government troops several times.

READ: South Sudan army battles rebels for key oil town

"We heard heavy shelling erupting early this morning," said Timothy Ngyuai from the aid agency CARE, adding they had taken initially been forced to take shelter in bunkers.

The army was not immediately available for comment.

The British ambassador in Juba, Ian Hughes, said the reports of fighting were "disappointing", coming just three days after UN Security Council envoys visiting the troubled nation warned both the government and rebel leaders of "consequences".

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"The situation there is desperate enough already," Hughes said. "Leaders need to control their fighters."

UN envoys met with President Salva Kiir in Juba and also spoke with rebel chief Riek Machar, urging them to stick to peace after three failed ceasefire deals.

US ambassador to the UN Samantha Power said on Tuesday there were "very worrying reports" more weapons and arms were being brought into South Sudan for a fresh offensive.

READ: UN Security Council South Sudan to push for peace

Looming famine

Thousands of people have been killed and more than 1.5 million have fled civil war sparked by a power struggle between Kiir and his sacked deputy Machar, with battles between government troops, mutinous soldiers and ragtag militia forces divided by tribe.

The UN has said the food crisis is the "worst in the world", with aid workers warning of famine within weeks if conflict continues.

READ: South Sudan famine within weeks without funding: aid agencies

UN aid chief in South Sudan Toby Lanzer said the fighting in Bentiu was a "stark reminder of the difficult conditions under which aid agencies and peacekeepers work".

More than 40,000 civilians are crammed into the UN base in Bentiu, flooded with heavy rains and knee-deep in raw sewage, in conditions the aid agency Doctors Without Borders (MSF) has described as "horrifying".

Over a third of residents are children younger than five. While MSF says that death rates have improved, a child is still dying in the camp each day.

Like almost 100,000 civilians in UN camps across South Sudan, the people fled into the base in December amid killings and massacres, and are now too fearful to return home with fighting still raging.

READ: Over 100,000 seeking shelter in South Sudan UN bases

The US and the European Union have already imposed penalties on three senior South Sudanese army commanders from the government and opposition, while nations in the regional bloc IGAD have suggested they could follow suit if progress was not made.

East African leaders are due to hold an IGAD meeting over the weekend in the Ethiopian capital to discuss the war in South Sudan.