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The World Health Organisation has warned of a "huge biological risk" after one of the warring parties in Sudan occupied the central public health laboratory in the capital Khartoum.
The warning cam as Western and Asian nations raced to mount rapid evacuation efforts from the country and violence punctured a fragile US-brokered ceasefire.
Gunfire and the roar of fighter jets was heard by CNN journalists in Khartoum on Tuesday, half a day after the announcement of a 72-hour truce raised hopes of opening up escape routes for civilians desperate to flee.
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The Sudanese army and the paramilitary group battling it for control of the country accused each other of violating the agreement.
Nima Saeed Abid, the WHO representative in Sudan, provided details of the laboratory occupation during a video briefing on Tuesday, describing it as "extremely dangerous".
"We have polio isolates in the lab, we have measles isolates in the lab, we have cholera isolates in the lab," he said, without saying which side had taken over the lab.
"So there is a huge biological risk associated with the occupation of the central public health lab in Khartoum by one of the fighting parties."
In an update sent to CNN, the WHO revealed "trained laboratory technicians no longer have access to the laboratory" while there have also been power cuts, meaning "it is not possible to properly manage the biological materials that are stored in the laboratory for medical purposes".
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The power cuts also mean there is a risk of spoilage of depleting stocks of blood bags, according to the director-general of the laboratory.
The United Kingdom, France, South Korea and a host of other countries confirmed on Tuesday they were pulling out nationals after US Secretary of State Antony Blinken announced that a three-day ceasefire had been agreed.
The White House is meanwhile considering a plan to send US troops to Port Sudan to help with the evacuation of American citizens, a US official with knowledge of the operations told CNN on Monday.
Two US warships are also being deployed toward Port Sudan, the USS Truxtun and the USS Lewis B Puller, according to the official.
Previous ceasefires have collapsed within hours of being struck since clashes first gripped Sudan in mid-April.
But the latest agreement, which Blinken said followed two days of "intense negotiation", had appeared to give countries a window in which to hurry citizens and staff to safety.
According to a statement from the Sudanese Armed Forces, Saudi Arabia was also involved in mediating the truce.
CNN's team in Djibouti obtained images released by the US military showing personnel arriving in that country. France and Pakistan both said they had evacuated hundreds of nationals, while China said most of its citizens had also been pulled out of the country.
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On Tuesday, both sides of the conflict accused each other of breaking the truce and there have been reports of heavy clashes north of the capital.
The armed forces said the paramilitary Rapid Support Forces (RSF) were moving military convoys to the capital in order to carry out a large scale military operation, had deployed snipers in parts of Khartoum, and were conducting operations near embassies. The army did not provide evidence for the claims.
The RSF blamed the army for violating the truce by "continuing to attack Khartoum by planes."
Eyewitnesses told CNN that fighter jets were heard over Omdurman to the north of the capital.
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Dwindling supplies in the capital
As the conflict rages on through its second week, water supplies are scarce and food is "running out" in Khartoum state, a witness told CNN on Tuesday.
"Shops are running out of food completely" and several food factories in the state had been looted, the witness, who asked to remain anonymous for security reasons, told CNN.
"As for the water supplies, we don't have water for the eleventh day continuously. We only get water from a well nearby.
"So you have to go all the way to the well with barrels or stuff if you have a car or stuff. If not you have to take something small to get enough water for you."
In Khartoum and surrounding areas including Wad Madani, the shortage of food, water, medicines and fuel are becoming extremely acute," the United Nations Office for the Coordination of Humanitarian Affairs said on Monday.
"Access to health care, including sexual and reproductive health care, has been critically impacted by the conflict," it said.
"Displacement of civilians continues to be reported in Khartoum, Northern, Blue Nile, North Kordofan, North Darfur, West Darfur and South Darfur states, as well as cross-border movements to surrounding countries."
Many Sudanese people caught in the middle of the fighting have attempted to make their own perilous escapes from the capital, taking advantages of brief breaks in combat to rush to safety.
Sudan has been racked with violence since a bloody power struggle between two rival generals spilled into the streets, with forces loyal to each man engaging in combat on the streets of Khartoum and in towns around the capital.
Over the course of fighting, the RSF and Sudanese military have issued statements discrediting one another, with unsubstantiated claims of their control over key posts of the capital and accusations of each side targeting civilians.
On Monday, the Sudanese military claimed that the RSF killed an Egyptian diplomat, while the RSF claimed the army targeted civilians in an airstrike on a Khartoum neighbourhood. Both groups did not provide evidence for the claims.
The West will eagerly watch developments on the ground on Tuesday, hoping that the current truce will stick.
"We're going to watch this as close as we can, monitor it as best we can and we're going to continue to urge both sides to abide by this ceasefire," White House National Security Council spokesperson John Kirby told CNN on Monday (Tuesday AEST).
When asked if the US has provided any incentives to dissuade the ceasefire from breaking down, Kirby told CNN's Wolf Blitzer, "We have made it very clear that they are responsible for the lives and livelihoods not only of the Sudanese people, but all the foreign nationals that are in Khartoum and in Sudan today, that they will bear that responsibility".
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