Fighting is continuing in areas of Sudan's capital, Khartoum, despite a ceasefire due to start at 18:00 local time (16:00 GMT).
In particular the battle has not stopped 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 to allow civilians to get medical help and supplies.
Residents are low on food and water.
Earlier on Tuesday, a woman living in Khartoum told the BBC that she has no more drinking water left in her home: "This morning we ran out."
Duaa Tariq said only one bottle remained, which was exclusively 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.
Nearly 200 people have been killed in the fighting which began on Saturday between the army and the Rapid Support Forces (RSF) paramilitary group.
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 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," Mr Griffith's 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.
