A 24-hour truce agreed between Sudan's rival military factions was quickly disrupted by renewed gunfire in the Khartoum capital region on Tuesday, despite U.S. pressure to calm fighting that has touched off a humanitarian crisis.

Heavy gunfire echoed in the background of live feeds by Arab television news channels and residents told Reuters that shooting had not ceased, with one saying they had heard an air strike being carried out in Omdurman, Khartoum's sister city on the opposite bank of the Nile river.

The conflict between Sudan's military leader and his deputy broke out four days ago, triggering what the United Nations has described as a humanitarian catastrophe, including the near collapse of the health system. At least 185 people have been killed in the fighting across the country.

The ceasefire that formally took effect at 6 p.m. (1600 GMT) would not extend beyond the agreed 24 hours, Army General Shams El Din Kabbashi, a member of Sudan's ruling military council, said earlier on al Arabiya TV.

U.S. Secretary of State Antony Blinken, speaking in Japan, said on Tuesday he had telephoned both army chief General Abdel Fattah al-Burhan and paramilitary leader General Mohamed Hamdan Dagalo, of the Rapid Support Forces (RSF), appealing for the 24-hour ceasefire "to allow the Sudanese to be safely reunited with families" and to provide them with relief.

Fighting appeared to tail off close to the deadline for the ceasefire, which coincided with the evening breaking of the daily fast during the Islamic holy month of Ramadan, but heavy gunfire reverberated around the capital region after 6 p.m.

A Reuters reporter in Khartoum said he heard tanks firing after the ceasefire was due to have started.

The source of shooting was unclear, though the RSF accused the military of violating the ceasefire within 15 minutes of it taking effect.

WARPLANES AND EXPLOSIONS

Earlier in the day the sounds of warplanes and explosions echoed across Khartoum. Residents in the neighbouring cities of Omdurman and Bahri reported air strikes that shook buildings and anti-aircraft fire. Fighting also raged in the west of the country, the United Nations said.

In video verified by Reuters, RSF fighters could be seen inside a section of the army headquarters in Khartoum. The fighters did not appear to control the sprawling site, a Reuters reporter in the capital said.

Burhan heads a ruling council installed after a 2021 military coup and the 2019 ouster of veteran autocratic president Omar Bashir during mass protests. Hemedti is Burhan's deputy on the ruling council.

Their power struggle has derailed an internationally-backed plan to shift to civilian democratic rule after decades of autocracy and military control in Sudan, which sits at a strategic crossroads between Egypt, Saudi Arabia, Ethiopia and Africa's volatile Sahel region.

Unless controlled, the violence also risks drawing in actors from Sudan's neighbourhood who have backed different factions, and could play into competition for regional influence between Russia and the United States.