A massive humanitarian and refugee crisis is emerging in Sudan as fighting between military factions intensifies.

So far, more than 420 people, including 264 civilians, have been killed in the conflict and almost 3,800 injured.

The sudden slide into violence between the Sudanese army and the Rapid Support Forces, a paramilitary group, has halted at times with fragile ceasefires called.

But the violence has displaced hundreds of thousands of locals and stranded thousands of foreigners, including diplomats and aid workers in the country – including about 100 Australians. The UK, US, France, Germany, Italy, Greece, Egypt, Saudi Arabia and Gulf states have all closing their embassies and have launched efforts to evacuate their nationals.

The UN refugee agency UNHCR says it is scaling up its assistance to people seeking safety in countries neighbouring Sudan, with the fighting set to trigger further displacement both within and outside the country.

The most significant cross-border movements in the region have been Sudanese fleeing to Chad, and South Sudanese refugees returning to South Sudan. There are also reports of people starting to arrive in Egypt.

“Some of the population movements we expect to observe in the coming days include outflows of new Sudanese refugees to neighbouring countries, returns of refugees who were being hosted by Sudan, and movements of other refugees being hosted by Sudan to other neighbouring countries,” a statement from the UNHCR said.

“UNHCR is working closely with partners and governments in the region to assess the needs of the newly arrived and to prepare a joint response. We thank them for continuing to keep their borders open to those fleeing Sudan – whether to seek international protection or to return to their countries of origin,” the statement said.

In Chad, UNHCR has deployed emergency teams along the eastern border with Sudan to respond to urgent needs for protection and humanitarian assistance.

The Government of Chad and UNHCR have started registrations to identify new arrivals and assess their needs.

Since the beginning of the fighting, at least 20,000 refugees have fled across the border into Chad. Many are located in villages only 5km away from Sudan, and more are expected to arrive in the coming days.

The UNHCR is planning to relocated displaced families to an existing refugee camp further from the border, while a new location is being identified to host additional arrivals.

More than 400,000 Sudanese refugees are already hosted across 13 camps and among local communities in eastern Chad.

UNHCR teams are also at border crossing points in South Sudan to monitor new arrivals and provide help.

So far, about 4,000 South Sudanese have crossed from Sudan.

UNHCR says many of those who have arrived have had the means to pay for transport from Khartoum and to continue their travel inside South Sudan.

Reports say a large number of fellow South Sudanese are trying to reach the border on foot.

There are more than 800,000 South Sudanese refugees in Sudan, a quarter of whom are in Khartoum and directly affected by the fighting.

But South Sudan is already suffering a major humanitarian crisis. The country has more than 2.3 million internally displaced people; almost three quarters of the population is in need of humanitarian assistance; and 2.2 million South Sudanese are refugees in neighbouring countries.

Aid agencies say they are concerned that a large number of new returns may destabilise already struggling local communities.

Sudan itself hosts more than a million refugees and 3.7 million internally displaced people and assistance programs were already overstretched.

In a strange twist of fate, there are reports that Rohingya, Yemini and Syrian refugees who have sought safety in Sudan are also caught up in the chaos.

Videos on social media platforms show Rohingya refugees as well as Yemenis and Syrians who are stuck in Sudan with no clear method of evacuation.

One Rohingya family is seen appealing for help, with the father of a family – with two young children and a wife – saying that the “situation is very bad”.

“We are fearful. We need a safe zone because here the situation is very bad with very heavy fighting,” the unidentified man says.

He is filmed carrying a placard that reads: “Please help us”.

Tension had been building for months between Sudan’s army and the RSF, which, acting together, toppled a civilian government in an October 2021 coup.

The friction was brought to a head by an internationally-backed plan to launch a new transition with civilian parties.

A final deal was due to be signed earlier in April, on the fourth anniversary of the overthrow of long-ruling autocrat Omar al Bashir in a popular uprising.

Both the army and the RSF were required to cede power under the plan.

But two sticking points emerged. Firstly a timetable for the RSF to be integrated into the regular armed forces and, secondly, that the army would be formally placed under civilian oversight.

When fighting broke out on 15 April, both sides blamed the other for provoking the violence.

The army accused the RSF of illegal mobilisation in preceding days and the RSF, as it moved on key strategic sites in Khartoum, said the army had tried to seize full power in a plot with Bashir loyalists.

The leaders of the two sides are General Abdel Fattah al-Burhan, head of the army and leader of Sudan’s ruling council since 2019 and his deputy on the council, RSF leader General Mohamed Hamdan Dagalo, commonly known as Hemedti.

Observers say that during the planning the transition, Hemedti aligned himself more closely with civilian parties from a coalition, the Forces for Freedom and Change (FFC), that shared power with the military between Bashir’s overthrow and the 2021 coup.

They say this was part of a strategy by Hemedti to transform himself into a politician.

Both the FFC and Hemedti, who has profited greatly from gold mining ventures, stressed the need to sideline Islamist-leaning Bashir loyalists and veterans who had regained a foothold following the coup and have deep roots in the army.

The latest conflict has damaged hopes that Sudan and its population of 46 million could emerge from decades of autocracy, internal conflict and economic isolation under Bashir.

And there are fears the conflict could destabilise an already volatile local region, which includes the Sahel, the Red Sea and the Horn of Africa.