The World Health Organization on Tuesday voiced concern about the "scale and speed" of an Ebola outbreak that has killed more than 130 people in the Democratic Republic of Congo and warned it could be lengthy.

The UN health agency has declared the surge of the highly contagious haemorrhagic fever in the east of the country an international health emergency.

No vaccine or therapeutic treatment exists for the Bundibugyo strain of Ebola which is responsible for the outbreak.

Ebola has killed more than 15,000 people in Africa in the past half-century.

Workers with shovels were seen digging to clear an abandoned Ebola treatment centre in the eastern city of Goma.

"We are currently spreading sand and preparing the site to help care for people affected by Ebola virus disease," said Philippe Jamaica, who was supervising the work.

Tonnes of emergency medical supplies, including infection prevention kits and tents, as well as experts have arrived in recent days, local WHO footage showed.

With the recent cases largely concentrated in difficult-to-access areas hit by long-running conflicts, few samples have been laboratory-tested and figures are based mostly on suspected cases.

Congolese Health Minister Samuel Roger Kamba told reporters on Tuesday that there had been 136 deaths suspected to be linked to Ebola and about 543 suspected cases, calling for international aid to help combat the spread.

WHO chief Tedros Adhanom Ghebreyesus said he was "deeply concerned about the scale and speed of the epidemic" while the agency's representative in the DRC said a vaccine candidate called Ervebo was being considered.

But Anne Ancia said it would likely take at least two months to be available, adding: "I don't think that in two months we will be done with this outbreak".

Conflict

At the hospital in Rwampara in northeastern Ituri province, the epicentre of the outbreak near the border with Uganda and South Sudan, the response was slow, despite the surge in cases.

A simple strip of plastic marked off the site reserved for receiving patients.

"We dig graves and bury people who died without gloves or any protection. We're so explosed," Salama Bamunoba, from a local youth organisation, told AFP.

"We're already at about 100 cases. We didn't have an appropriate place to do triage and isolate suspected cases" until Monday, said one local hospital official.

Kamba told reporters that equipment was being delivered. "We have everything healthcare providers will need," he added.

The DRC's deadliest Ebola outbreak, between 2018 and 2020, claimed nearly 2,300 lives from 3,500 cases.

The east is a gold-mining hub with people regularly crisscrossing the region, and it has been plagued by clashes between local militias for years.

The virus has already spread into neighbouring provinces, as well as beyond the DRC's borders into Uganda.

The Ebola outbreak is the 17th in the vast central African country of more than 100 million people.

Vaccines are only available for the Zaire strain of the disease, which has caused the biggest recorded outbreaks.

The Bundibugyo strain has previously been responsible for outbreaks in Uganda in 2007 and in the DRC in 2012. The mortality rate was 30 to 50 percent.

Congolese President Felix Tshisekedi on Tuesday urged citizens to keep "calm" and take precautions, the presidency said on X.

Suspected cases have been reported in the commercial hub of Butembo in North Kivu province, some 200 kilometres (125 miles) from the epidemic's ground zero, Kamba said.

Another case has been recorded in Goma, the North Kivu provincial capital, which was seized by M23 fighters in January last year.

Congolese doctor Denis Mukwege, who won the Nobel Peace Prize in 2018, appealed to the Rwanda-backed group to reopen the city's airport to help combat the outbreak.

The front line dividing the anti-governmental group and government forces runs through the provinces of North Kivu and neighbouring South Kivu.

"Humanitarian access and coordination between the various stakeholders, particularly the parties to the conflict, could be one of the challenges for the response," Francois Moreillon, of the International Committee of the Red Cross in the DRC, told AFP on Monday.

He called on the armed groups to guarantee effective access, cooperation and humanitarian coordination.

US screening

The government in neighbouring Uganda said two Ebola cases -- one infection and one death -- had been recorded there, involving Congolese nationals who crossed the border.

Germany meanwhile said on Tuesday it was readying to receive and treat a US citizen who has contracted the virus -- a doctor from an American Christian NGO.

The United States announced it was screening air passengers from outbreak-hit areas and temporarily suspending visa services.

It also urged citizens to avoid travel to the DRC, South Sudan and Uganda, and to reconsider travel to Rwanda.