Fighting disrupts the US-brokered ceasefire as the WHO issues a "biological risk" warning after Sudanese fighters seize a facility.

After Sudanese rebels took control of the National Public Health Laboratory in the nation's capital Khartoum, the World Health Organization issued a "huge biological risk" warning on Tuesday. This came as foreign countries rushed to launch quick evacuation operations from the country and violence shattered a tenuous ceasefire mediated by the US.

In Khartoum on Tuesday, half a day after the announcement of a 72-hour ceasefire raised hopes of opening up escape routes for desperate civilians to evacuate, CNN correspondents heard gunfire and the thunder of fighter jets. Eyewitnesses informed CNN that fierce fighting broke out between the Sudanese military and the Rapid Support Forces, the paramilitary group opposing the army for control of the nation. The fighting took place in the northern region of Khartoum state.

On Tuesday, half a day after the proclamation of a 72-hour ceasefire raised hopes of opening up escape routes for civilians anxious to flee, our correspondents in Khartoum heard gunfire and the thunder of fighter jets. In the northern region of Khartoum state, heavy fighting broke out between the Sudanese military and the Rapid Support Forces, the paramilitary group opposing the army for control of the country, eyewitnesses told our journalists. 

The two belligerent parties each accused the other of breaking the agreement.

The World Health Organization also reported on Tuesday that there have been at least 459 deaths and at least 4,072 injuries in Sudan since the start of the unrest eleven days ago.

'Germ bomb' potential in the lab that was seized.

A senior medical source informed our journalists that RSF forces had taken control of the lab, which has samples of illnesses and other biological material. The WHO stated that medical technicians no longer had access to the facility but did not assign blame for the lab seizure.

The WHO official for Sudan, Nima Saeed Abid, called the situation "extremely dangerous because we have polio isolates in the lab, measles isolates in the lab, and cholera isolates in the lab."

The key public health lab in Khartoum being occupied by one of the combatants poses a significant biological concern, he continued.

In a statement to our correspondents, the World Health Organisation said that trained laboratory technicians are no longer able to access laboratories because they've suffered from power cuts and it is impossible to properly maintain biological materials stored in labs for medical purposes.

The director-general of the laboratory claims that the risk of spoiling and running out of blood bags is also increased by the power outages.

According to the medical source, "the danger lies in the outbreak of any armed confrontation in the laboratory because that will turn the laboratory into a germ bomb."

We are facing a genuine biological threat, the insider continued, so an immediate and quick international involvement is necessary to restore electricity and secure the laboratory from any armed confrontation.