By Khalid Abdelaziz and Nafisa Eltahir
DUBAI (Reuters) -Clashes between rival military factions broke out on Wednesday in Sudan's capital Khartoum, residents said, threatening to shatter a fragile ceasefire designed to allow for the delivery of aid and create conditions for a more lasting truce.
The ceasefire deal, which is being monitored by Saudi Arabia and the United States as well as the warring parties, was reached after five weeks of warfare in Khartoum and outbursts of violence in other parts of Sudan, including the western region of Darfur.
The fighting pits Sudan's army against the powerful paramilitary Rapid Support Forces (RSF) and has worsened a humanitarian crisis, forced over 1.3 million people to flee their homes and threatened to destabilise the wider region.
The ceasefire had brought a relative lull in fighting in Khartoum on Tuesday, although little sign of a rapid increase in humanitarian relief.
Witnesses reported clashes in several areas of the capital on Wednesday afternoon.
West of central Khartoum columns of black smoke could be seen rising into the air, and there was shelling near an army camp in southern Khartoum, they said.
In Bahri, one of the three cities around the confluence of the Blue Nile and White Nile rivers that make up Sudan's greater capital, the sound of clashes and artillery fire could be heard.
Earlier, residents reported artillery fire near the Wadi Sayidna military base on the outskirts of Omdurman, the third city.
The ceasefire was agreed to on Saturday following talks in Jeddah mediated by Saudi Arabia and the United States. Previous ceasefire announcements have failed to stop the fighting.
In statements late on Wednesday, the army and RSF accused each other of violating the agreement.
The RSF said it was forced to defend itself against land, artillery and air strikes by the army. The army in turn accused the RSF of attacks on the country's mint, army airbases and several cities west of the capital.
Witnesses in Omdurman reported that an army fighter plane had been shot down, and videos posted on social media appeared to show the incident. The footage could not immediately be verified.
The army said the plane had malfunctioned, while the RSF said it had brought down the plane.
In Nyala, the capital of South Darfur State, days of clashes between the army and the RSF had left most of the main market burned down, two residents said.
"We're in a very difficult situation. We feel emotionally broken and terrified," said resident Malak Ibrahim, adding that her family hadn't had regular access to water for two weeks.
Activists in Zalingei, the capital of Central Darfur State, said RSF-backed militias had surrounded the city and started looting homes and businesses. Zalingei and West Darfur State capital El Geneina, where hundreds have been killed since last month, both appeared to be cut off from phone networks.
The conflict in Sudan erupted in mid-April as plans for an internationally backed political transition toward elections under a civilian government were set to be finalised, bringing sustained air strikes and ground fighting to Khartoum for the first time. Many residents are struggling to survive as they face prolonged water and power cuts, a collapse of health services and widespread lawlessness and looting.
The United Nations human rights chief called the situation in Sudan "heartbreaking" and said there were "very deeply troubling" accounts of sexual violence in Khartoum and Darfur with at least 25 cases reported so far and the real number likely much higher.
Aid workers said many of the supplies and staff arriving at Port Sudan on the Red Sea coast have been awaiting security permits and guarantees.
U.N. humanitarian agency OCHA said agencies were ready to deliver aid to more than 4 million people.
"I therefore urge the relevant authorities to take all steps possible to ensure they provide the security environment and (ease) bureaucratic impediments at local and national level," Abdou Dieng, the U.N. humanitarian coordinator in Sudan, said in a statement.
Sudan was facing severe humanitarian pressures even before the conflict broke out.
More than 1 million people have been displaced within Sudan and 319,000 have fled Sudan to neighbouring countries, some of which are similarly impoverished and have a history of internal conflict, according to the International Organization for Migration. Many have crossed into Chad and Egypt in the last few days, said Filippo Grandi, head of the U.N. refugee agency.
"We need more resources, urgently, to support countries hosting refugees," he said on Twitter.
The U.N. says that the number of people requiring aid within Sudan has jumped to 25 million, more than half the population.
(Reporting by Khalid Abdelaziz in Dubai and Nafisa Eltahir in Cairo; Additional reporting by Emma Farge in Geneva; Writing by Aidan Lewis; editing by Christina Fincher, Mark Heinrich, Mark Porter, Grant McCool and Cynthia Osterman)