KHARTOUM, SUDAN -- Sudan's military ruled out negotiations with a rival paramilitary force on Thursday, saying it would only accept its surrender as the two sides continued to battle in central Khartoum and other parts of the country, threatening to wreck international attempts to broker a longer ceasefire.

A tenuous 24-hour ceasefire that began the previous day ran out Thursday evening with no word of an extension. The military's statement raised the likelihood of a renewed surge in the nearly weeklong violence that has killed hundreds and pushed Sudan's population to the breaking point. Alarm has grown that the country's medical system was on the verge of collapse, with many hospitals forced to shut down and others running out of supplies.

The truce had failed to put a stop to fighting throughout the day and brought only marginal calm to some parts of the capital, Khartoum. But many residents took advantage to flee the homes where they have been trapped for days. "Massive numbers" of people, mostly women and children, were leaving in search of safer areas, said Atiya Abdulla Atiya, secretary of the Doctors' Syndicate.

Thursday afternoon, the military said in a statement that it would not negotiate with its rival, the Rapid Support Forces, and would only discuss the terms of its surrender. "There would be no armed forces outside (of) the military system," it said.

The demise of the truce, the second attempt this week, underscored the failure of the United States, UN, European Union and regional powers to push Sudan's top generals to halt their campaigns to seize control of the country. Instead, army chief Gen. Abdel Fattah Burhan and RSF commander Gen. Mohammed Hamdan Dagalo each appear determined to win outright military victory over the other.

In a sign they expect violence to escalate, the U.S. and other countries were making preparations to evacuate their citizens in Sudan -- a difficult prospect since most major airports have become battlegrounds and movement out of Khartoum to safer areas is dangerous.

The U.S. military is moving assets to a base in the Horn of Africa nation of Djibouti for a possible evacuation of American Embassy personnel, administration officials said. Japan plans to send military planes to Djibouti, and the Netherlands has dispatched its own to Jordan.

UN Secretary-General Antonio Guterres appealed for the combatants to commit to a three-day ceasefire to coincide with the holiday of Eid al-Fitr, beginning Friday, marking the end of the Islamic holy month of Ramadan. "We are living a very important moment in the Muslim calendar. I think this is the right moment for a ceasefire to hold," he told reporters.

But so far direct communications to the rival generals by U.S. Secretary of State Antony Blinken, the Turkish president and others over the past days have been unable to secure even 24 hours of calm, much less a longer truce or negotiations to resolve the crisis. Each side's main regional allies, Egypt and the United Arab Emirates, have called in vain for talks.

At least 330 people have been killed and 3,300 wounded in the fighting since it began Saturday, the World Health Organization said, but the toll is likely higher because many bodies lie uncollected in the streets.

Through the day Thursday, gunfire could be heard constantly across Khartoum. Residents reported the heaviest fighting around the main military headquarters in central Khartoum. Military warplanes struck RSF positions at the airport and in the neighbouring city of Omdurman, residents said. The military said its warplanes also struck a convoy of RSF vehicles heading to the capital, though the claim could not be independently confirmed.

Khartoum residents have been desperate for a respite after days of being trapped in their homes, with food and water running out.

Aid groups have been unable to deliver help to Sudan's overwhelmed hospitals, Atiya said. Hospitals in Khartoum are running dangerously low on medical supplies, often operating without power and clean water. Around 70% of hospitals throughout the country are out of service because of the fighting, the Sudanese Doctors Syndicate said, adding that at least nine hospitals were bombed.

The two sides are battling elsewhere in the country as well, and there were reports of heavy fighting in the city of Obeid, some 400 kilometres (250 miles) southwest of Khartoum.

The Doctors' Syndicate said at least 26 civilians and 17 police were killed in Obeid, and that four hospitals and a church were damaged, some by airstrikes. Two markets were looted, and more than 3,300 people fled their homes, with many sheltering in a school and a sports facility, it said.

The fighting has been disastrous for a country where the United Nations says around a third of the population -- some 16 million people -- are in need of humanitarian aid. The UN children's agency UNICEF warned that critical care has been disrupted for 50,000 severely acutely malnourished children, who need round-the-clock treatment.

Save the Children said power outages across the country have destroyed cold chain storage facilities for lifesaving vaccines, as well as the national stock of insulin and several antibiotics. Millions of children, the aid group said, are now at risk of disease and further health complications. It said 12% of the country's 22 million children are suffering from malnutrition and are vulnerable to other diseases.

The Egyptian and Sudanese militaries said that Egypt succeeded in repatriating dozens of its military personnel who had been detained by the RSF when it attacked Merowe airport, north of the capital, early in the fighting. Egypt said its personnel were there for training and joint exercises.

The conflict has once again derailed Sudan's attempt to establish democratic rule after a popular uprising helped depose longtime autocrat Omar al-Bashir four years ago. Burhan and Dagalo jointly carried out a coup purging civilians from a transitional government in 2021.

The explosion of violence came after weeks of growing tensions between the two generals over a new international push for a return to civilian government.

Both sides have a long history of human rights abuses. The RSF was born out of the Janjaweed militias, which were accused of widespread atrocities when the government deployed them to put down a rebellion in Sudan's western Darfur region in the early 2000s.

The conflict has raised fears of a spillover from the strategically located nation to its African neighbours.

Sudan's fighting has caused up to 20,000 Sudanese to seek refuge in eastern Chad, the UN said Thursday. At least 320 Sudanese soldiers fled to Chad, where they were disarmed, said Daoud Yaya Brahim, Chad's defence minister. The troops were apparently fleeing from Darfur, where the RSF is the most powerful armed force.

"Chad is for the moment trying to remain neutral ... (but) Chad will be forced to pick sides if Sudan continues its descent into civil war," said Benjamin Hunger, Africa analyst for Verisk Maplecroft, a risk assessment firm.

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Magdy reported from Cairo. Associated Press correspondent Fay Abuelgasim in Beirut contributed to this report.