By Mohamed Nureldin and Khalid Abdelaziz
KHARTOUM (Reuters) -Artillery fire could be heard in parts of Khartoum and warplanes flew overhead on Tuesday, residents said, though an internationally monitored ceasefire appeared to have brought some respite from heavy fighting in the Sudanese capital.
Night-time airstrikes were reported in at least one area after the ceasefire started late on Monday, but residents otherwise reported relative calm.
The truce was agreed at talks in Jeddah on Saturday after five weeks of fierce battles between the army and the paramilitary Rapid Support Forces (RSF). It is being tracked by Saudi Arabia and the United States and is meant to allow for the delivery of humanitarian relief.
The two countries said in a joint statement late on Tuesday that preparations had begun for urgently needed humanitarian relief operations.
Sudanese activists wrote to the United Nations envoy to Sudan welcoming the ceasefire agreement but complaining of severe human rights abuses against civilians that they said took place as the fighting raged and should be investigated.
Volunteer groups that have been at the forefront of local aid efforts in the capital were preparing to receive supplies, though much of the aid that has arrived in Port Sudan on the Red Sea coast is yet to be distributed as agencies wait for security clearance, activists and aid workers said.
Medical humanitarian group MSF, which runs projects in 10 states in Sudan, said there had been violence in parts of the country including several cities in the western region of Darfur.
Sudan's health ministry said in a statement that the RSF raided and occupied the Ahmed Qassim hospital in Bahri just before the ceasefire and had stationed itself in another Bahri hospital, Alban Jadeed, on Tuesday morning. The RSF accused the health ministry of publishing "lies".
The ceasefire deal has raised hopes of a pause in a war that has driven nearly 1.1 million people from their homes, including more than 250,000 who have fled to neighbouring countries.
"Our only hope is that the truce succeeds, so that we can return to our normal life, feel safe, and go back to work again," said Khartoum resident Atef Salah El-Din, 42.
Although fighting has continued through previous ceasefires, this was the first to be formally agreed following negotiations and the first to include a monitoring mechanism.
U.S. Secretary of State Antony Blinken said the monitoring mechanism would be "remote", without giving details.
"The Jeddah talks have had a narrow focus. Ending violence and bringing assistance to the Sudanese people. A permanent resolution of this conflict will require much more," Blinken said in a video message.
The U.S.-Saudi joint statement said the two Sudanese factions had failed to abide by commitments not to seek military advantage in the days before the truce began, and the monitoring committee was seeking to verify reported violations since it went into effect on Monday.
The U.S. will press the combatants to "stop the violence when we see violations of the ceasefire" and will use unspecified "additional tools" if appropriate, State Department spokesperson Matthew Miller told reporters in Washington.
Since the start of the conflict on April 15, Sudanese activists have complained of indiscriminate shelling and airstrikes against residential areas as well as the taking of civilians as human shields, extrajudicial killings, torture and sexual violence.
MSF said violence, looting and administrative and logistical challenges had continually hampered efforts to increase its activities.
"We are experiencing a violation of humanitarian principles and the space for humanitarians to work is shrinking on a scale I've rarely seen before," said Jean-Nicolas Armstrong Dangelser, MSF's emergency coordinator in Sudan.
The crisis is also putting pressure on Sudan's neighbours.
Sudanese refugees are streaming into Chad so quickly that it will be impossible to relocate them all to safer places before the start of the rainy season in late June, a senior Red Cross official said on Tuesday, flagging the risk of a disaster.
Some 60,000-90,000 people have fled into neighbouring Chad, the U.N. refugee agency said this week.
(Reporting by Mohamed Nureldin in Khartoum, Khalid Abdelaziz and El Tayeb Siddig in Dubai; Adam Makary and Aidan Lewis in Cairo and Emma Farge in Geneva; Additional reporting by Jonathan Landay and Simon Lewis in Washington; Writing by Michael Georgy, Aidan Lewis and Cynthia Osterman; Editing by Bernadette Baum, William Maclean and Grant McCool)