Army officers take rest near a checkpost in Khartoum. File

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.

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 on 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.

The United States said on Thursday 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."

"Once the forces make clear by their actions that they are serious about complying with the ceasefire, the United States and the Kingdom of Saudi Arabia are prepared to resume facilitation of the suspended discussions to find a negotiated solution to this conflict," a State Department spokesperson said.

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.

One witness said there was "heavy artillery fire from army camps" in the capital's north. Another reported "artillery blasts on the RSF camp in Al Salha" in southern Khartoum — the largest paramilitary base and arsenal in the city.

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."