Airstrikes and reports of renewed fighting have threatened a delicate three-day truce in Sudan, while a senior aid worker warned of a potential “huge biological hazard” resulting from the armed seizure of a Khartoum laboratory containing deadly diseases. A 72-hour ceasefire came into effect across the country at midnight on Monday night and was largely holding. It is intended to give Sudanese people respite from days of bloodshed and allow the wounded to reach already limited medical care. World powers hope it will also provide time for a massive international rescue mission to fly out evacuees. Three previously attempted ceasefires have failed over 11 days of fighting. So far, at least 459 people have been killed and more than 4,000 wounded, according to UN agencies. On Tuesday morning, airstrikes struck the city of Omdurman, a city across the Nile from Khartoum, with a least one bomb hitting a civilian home. Later in the day, a private clinic in the city was hit by an anti-aircraft rocket, injuring 10 people, and clashes were heard spreading to parts of north Khartoum. Somaia Hassan, a mother of three, said she was hiding under a bed and citing verses from the Qur’an as gunfire trapped her in her home. The violence has pitted army units loyal to its military ruler, Gen Abdel Fattah al-Burhan, against the Rapid Support Forces (RSF) led by Mohamed Hamdan Dagalo, known as Hemedti. The UN secretary general, António Guterres, has said the fighting could “engulf the whole region and beyond”. “We must all do everything within our power to pull Sudan back from the edge of the abyss,” Guterres said on Monday. There have been reports of militias from neighbouring Chad joining the conflict on the side of the RSF, with gunmen arriving on motorbikes. An accountant living in El Geteina, on the border with Chad, said hospitals were closed due to a lack of medicine. “Now the number of the killed and injured people is unknown,” said Issmat Brahim. “I believe they are dozens if not hundreds, and the death toll will increase.” The World Health Organization’s representative to Sudan, Nima Saeed Abid, said local technicians could not access the national public health laboratory. The centre held samples of measles, cholera and polio pathogens and other hazardous materials, he said. Fighters “kicked out all the technicians from the lab … which is completely under the control of one of the fighting parties as a military base,” Abid said, declining to specify which warring side had seized the facility. “There is a huge biological risk associated with the occupation of the central public health lab. This is the main concern: no accessibility to the lab technicians to go to the lab and safely contain the biological material and substances available.” Clashes have paralysed hospitals and other essential services and left many residents stranded in their homes with dwindling food and water supplies. The UN humanitarian office (OCHA), which coordinates relief efforts, has been forced to cut back on some of its activities due to the violence. At least five aid workers have been killed since fighting broke out, and the International Organization for Migration and the World Food Programme have suspended some activities after losing staff. “In areas where intense fighting has hampered our humanitarian operations, we have been forced to reduce our footprint,” said Jens Laerke, a spokesperson for the OCHA. “But we are committed to continue to deliver for the people of Sudan.” The sudden departure of foreigners and closure of embassies has prompted fears in Sudan that international powers expect a worsening fight and are prioritising their diplomats and citizens. Western officials say they are trying to end the hostilities through diplomacy. Patrick Youssef, the International Committee of the Red Cross (ICRC) regional director for Africa, has urged other countries to continue to put pressure on Sudan to find a “long-lasting solution”.