The World Health Organisation warned Tuesday of a “huge biological risk” after Sudanese fighters seized the National Public Health Laboratory in the capital Khartoum, as foreign 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. Heavy clashes erupted between the Sudanese military and the Rapid Support Forces, the paramilitary group battling the army for control of the country, in the northern part of Khartoum state, eyewitnesses told CNN.

The two warring sides accused each other of violating the agreement.

The number of people killed in Sudan since violence broke out eleven days ago has reached at least 459 deaths, the World Health Organisation also said on Tuesday, with at least 4,072 people injured.

A high-ranking medical source told CNN that the lab, which contains samples of diseases and other biological material, had been taken over by RSF forces. The WHO did not appoint blame for the lab seizure but said medical technicians no longer had access to the facility.

Nima Saeed Abid, the WHO representative in Sudan, described the development as “extremely dangerous because we have polio isolates in the lab, we have measles isolates in the lab, we have cholera isolates in the lab.”

“There is a huge biological risk associated with the occupation of the central public health lab in Khartoum by one of the fighting parties,” he added.

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The WHO said in a statement to CNN that “trained laboratory technicians no longer have access to the laboratory” and that the facility had suffered 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 medical source told CNN that “the danger lies in the outbreak of any armed confrontation in the laboratory because that will turn the laboratory into a germ bomb.”

“An urgent and rapid international intervention is required to restore electricity and secure the laboratory from any armed confrontation because we are facing a real biological danger,” the source added.

CNN has reached out to the RSF for comment.

The United Kingdom, France, South Korea and a host of other countries confirmed Tuesday that 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.

Three US warships are also being deployed off the coast of Sudan. A Navy official told CNN that the US is sending the USNS Brunswick to Sudan, one day after the Pentagon said the USS Truxton is already off the coast of the country and the USS Lewis B. Puller is on its way.

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.

As many as 500 people fleeing the fighting have begun boarding the French frigate “Lorraine” in Port-Sudan on Tuesday afternoon, a spokesman for the French Chief of Defense Staff told CNN.

At least one US citizen, Dr. Bushra Ibnauf Sulieman, a Sudanese American physician, was killed in Khartoum on Tuesday.