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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, which is in the city centre and 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 She 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 so far.
The UN aid chief has also warned of reports which 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.
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