•Fighting was reported around the army headquarters by the airport in the city centre, which is surrounded by residential areas.
•Residents are low on food and water as clashes between the army and the Rapid Support Forces (RSF) paramilitary group continue.
Heavy gunfire and the roar of warplanes have shattered plans for a ceasefire in the Sudanese capital Khartoum, which was due to start at 18:00 (16:00 GMT).
Fighting was reported around the army headquarters by the airport in the city centre, which is surrounded by residential areas.
Two rival generals at the heart of the conflict had agreed to a 24-hour humanitarian pause.
Nearly 200 people have been killed in the fighting which began on Saturday.
Residents are low on food and water as clashes between the army and the Rapid Support Forces (RSF) paramilitary group continue.
On Tuesday, UN Secretary General António Guterres' spokesman said, "The fighting in Sudan, including Khartoum and various other locations, is continuing. No sign of real abatement of the fighting."
Earlier in the day, a woman living in Khartoum told the BBC that she had no more drinking water left in her home.
Duaa Tariq said only one bottle remained, which she was saving for her two-year-old child, as her family crammed into a "tiny corridor" to avoid gunfire.
"Most of the people [that] died, died in their houses with random bullets and missiles, so it's better to avoid exposed places in the house" like windows, Ms Tariq said.
At the University of Khartoum, a student was killed after being hit by a stray bullet.
"We were going to get food for the rest of the students," law student Mosaab Sharif, who is sheltering in a building near the campus, told the BBC.
A Facebook post, verified by the BBC, said the body had been buried on campus after safe passage off site could not be secured.
"There were three of us, and then he was hit in the chest. We couldn't even help him. As we were burying our colleague, one of us was hit with a bullet in his hand," Mr Sharif added.
He said that "snipers have been targeting anyone with flash lights".
Half an hour before the ceasefire was due to start, Khartoum residents were shocked to hear that three children - brothers living in the east of the city - had been killed in a bombardment.
Residents broke their Muslim Ramadan fast just after 18:00 local time to the sound of gunfire, with eyewitnesses in Bahri, in the north of the city, saying aircraft were flying overhead.
Another woman in Khartoum told the BBC that heavy weapons fire had continued well after the ceasefire was due to come into effect.
She described how earlier in the day she had escaped with her one-year-old child from her home as it was being struck by missiles.
Even if the fighting does die down in the next 24 hours, it is unlikely to be enough time for civilians to seek help, with the Red Cross saying the health system is on the verge of collapse.
The aid group said it has been receiving multiple calls for help from people trapped in their homes in a city that has an estimated population of 10 million residents, with most struggling to cope without electricity.
Fighting has also been taking place elsewhere in Sudan, including in Darfur to the west.
The UN aid chief has warned of reports that say humanitarian workers are being attacked and sexually assaulted.
"This is unacceptable and must stop," Martin Griffiths tweeted, after the time the ceasefire was expected to have been implemented.
The fighting is between army units loyal to the de facto leader, Gen Abdel Fattah al-Burhan, and the RSF, a notorious paramilitary force commanded by Sudan's deputy leader, Mohamed Hamdan Dagalo, better known as Hemedti.