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A US-brokered ceasefire between Sudan's warring generals entered its second day on Wednesday but remained fragile after witnesses reported fresh air strikes and paramilitaries claimed to have seized a major oil refinery and power plant.

"The pause was not fully upheld, with attacks on headquarters, attempts to gain ground, air strikes, and explosions in different areas of the capital," UN Special Representative Volker Perthes told the Security Council on Tuesday.

Perthes said he maintained contact with both generals: army chief Abdel Fattah al-Burhan and his deputy-turned-rival, Mohamed Hamdan Daglo, who commands the heavily armed paramilitary Rapid Support Forces (RSF).

"There is yet no unequivocal sign that either is ready to seriously negotiate," Perthes said.

Security fears were compounded when the World Health Organization (WHO) warned Tuesday of a "huge biological risk" after fighters occupied a Khartoum laboratory holding samples of cholera, measles, polio and other infectious diseases.

The fighting has killed hundreds of people and left some neighbourhoods of greater Khartoum in ruins, prompting thousands of foreigners and Sudanese to flee.

'Children horrified'

As combat eased in the city of five million, foreign governments have been organising road convoys, aircraft and ships to get thousands of their nationals out.

A boat carrying nearly 1,700 civilians from more than 50 countries arrived in Saudi Arabia early Wednesday, the kingdom's foreign ministry said.

It added that it has evacuated 2,148 people, including more than 2,000 foreigners.

Other evacuation efforts continued, with a British military transport plane landing in Cyprus.

"The most difficult thing was the sounds of the bombing and the jet fighters while flying above our home. That horrified the children," said Safa Abu Taher, who landed with her family at a military airport in Jordan Tuesday night.

Bewildered civilians were seen walking down one street in Khartoum North where almost all buildings were blasted out and smoke rose from scorched ruins, in unverified video posted on social media.

Witnesses in the same area later reported air strikes, and paramilitary forces firing anti-aircraft weapons.

Late Tuesday, witnesses reported more air strikes in Khartoum North where they said fighter jets struck RSF vehicles.

Jailbreak reportsÌý

The RSF posted a video in which it claimed to be in control of an oil refinery and the associated Garri power plant more than 70 kilometres (43 miles) north of Khartoum.

Shortly before, the army had warned in a Facebook post of "heavy movement towards the refinery in order to take advantage of the truce by taking control of the refinery".

The two sides have both made unverifiable claims to control key sites, adding to what experts call an overwhelming state of fear in the capital.

According to lawyers, at least one jailbreak took place earlier this week, with reports of another at Kober prison, where former dictator Omar al-Bashir -- who is wanted by the International Criminal Court for war crimes, crimes against humanity and genocide -- was being held.

Ahmed Harun, a former top aide who is also wanted by the ICC, said in a recorded address to Sudanese television on Tuesday that a number of officials from Bashir's regime were out of jail.

"We remained in our detention at Kober, under the crossfire of this current battle, for nine days," even after the jail was emptied of both guards and prisoners, and "have now taken responsibility for our protection in our own hands" in another location.

Bashir's whereabouts could not be independently verified.

In Wad Banda, West Kordofan state, witnesses reported clashes between the army and RSF, including the use of fighter jets.

In West Darfur, "near the Chadian border, fighting has resumed with increased and worrying reports of tribes arming themselves and joining the fight," Perthes said Tuesday, adding that "intercommunal clashes" have also broken out in Blue Nile, on the southeastern border with Ethiopia.

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