Shelling and aerial bombardments killed 18 civilians at a Khartoum market as fighting showed no signs of abating Thursday and the US warned it could act against Sudan's rival leaders.

For more than six weeks, Khartoum and other parts of the country have been gripped by bloody warfare between the army and the paramilitary Rapid Support Forces.

Fighting continued Thursday, with witnesses reporting "heavy artillery" in the capital's north.

The army on Wednesday blasted RSF bases in Khartoum after pulling out of the talks in the Saudi city of Jeddah, accusing its rival of violating a ceasefire that was meant to allow aid deliveries.

"Eighteen civilians were killed and 106 wounded" by army artillery fire and aerial bombardments Wednesday on a market in southern Khartoum, a committee of human rights lawyers said.

The toll was confirmed by a neighbourhood group that organises aid, which said the situation was "catastrophic" and appealed for help from doctors and for blood donations.

US Secretary of State Antony Blinken warned that the United States could take action against rival Sudanese leaders after the collapse of a US- and Saudi-brokered truce.

Washington is "looking at steps that we can take to make clear our views on any leaders for taking Sudan in the wrong direction," Blinken told reporters at NATO talks in Oslo.

Earlier, a US State Department spokesperson said there had been "serious violations of the ceasefire by both sides" and warned it would only be ready to mediate between the warring parties when they get "serious".

In both north and south Khartoum on Wednesday, troops loyal to army chief Abdel Fattah al-Burhan attacked key bases of the RSF led by commander Mohamed Hamdan Daglo, residents told AFP.

- 'Fight until victory' -

One witness said there was "heavy artillery fire from army camps" in the city's north.

Another reported "artillery blasts on the RSF camp in Al-Salha" in southern Khartoum -- the largest paramilitary base and arsenal in the capital.

The attacks came two days after US and Saudi mediators said the two sides had agreed to extend by five days the initial week-long humanitarian truce.

Mediators admitted the truce had been "imperfectly observed", but said the extension would "permit further humanitarian efforts".

The army walked out "because the rebels have never implemented a single one of the provisions of a short-term ceasefire which required their withdrawal from hospitals and residential buildings", a Sudanese government official said.

The health ministry published a list Thursday of 34 health facilities that "the rebel militia continues to use as army barracks" and has not vacated "despite the truce".

Despite repeated pledges from both sides, fighting has flared this week both in greater Khartoum and in the western region of Darfur.

"The army is ready to fight until victory," Burhan declared during a visit to troops in Khartoum.

The RSF said they would "exercise their right to defend themselves", and accused the army of violating the truce.

- Looting of food -

Since fighting erupted on April 15, more than 1,800 people have been killed, according to the Armed Conflict Location and Event Data Project.

The UN says 1.2 million people have been internally displaced and more than 425,000 have fled abroad.

World Food Programme chief Cindy McCain condemned Thursday "the looting of WFP food and assets happening right now in El Obeid", 350 kilometres (220 miles) southwest of Khartoum.

Medics and humanitarian agencies have reported repeated looting since the start of the conflict, which has killed 18 aid workers.

"Food for 4.4 million people is at stake," McCain wrote on Twitter.

More than half the population -- 25 million people -- are now in need of aid and protection, the UN says.

Entire districts of Khartoum no longer have running water, electricity is only available for a few hours a week, and three quarters of hospitals in combat zones are not functioning.

Hundreds have been killed in Darfur, on Sudan's western border with Chad, the United Nations said.

The UNHCR said Thursday more than 100,000 Sudanese refugees had fled to Chad since April, and called for emergency financial support.

Darfur has never recovered from the years-long war that began in 2003 when a rebel uprising led strongman Omar al-Bashir to unleash the Janjaweed militia, from which the RSF are descended.

Experts say Burhan is facing increasing pressure from his own Islamist supporters and remnants of the Bashir regime, with whom he had built a symbiotic relationship in order to gain power.