Six tons of medical supplies to fight Ebola are set to arrive in the Democratic Republic of Congo (DRC) on Tuesday, but limited testing capacity was slowing the response to the outbreak, a World Health Organization (WHO) official said.

There have been at least 500 suspected cases and 131 suspected deaths from Ebola, which has also spread to neighbouring Uganda, according to the WHO.

"We have sent 12 tons of supply. An additional six are arriving today. These include personal protective equipment for frontline health workers (and) samples," said Anne Ancia, WHO Representative in Congo.

Ancia said there was "great uncertainty" about the extent and scale of the outbreak of the Bundibugyo strain of Ebola, and efforts were underway to increase surveillance, testing and contact tracing.

She said testing for the Bundibugyo strain was limited, with only six tests an hour possible. The outbreak took weeks to detect, she said, partly because tests used in the outbreak zone were designed for the more common Zaire strain.

"The surveillance capacity and investigation capacity are very limited in this region in general," she said.

BioFire Defense, an affiliate of French diagnostic firm bioMerieux, makes an FDA-cleared test — BioFire Global Fever Special Pathogens Panel — that can detect multiple Ebola species, including Bundibugyo.

A company spokesperson said it is increasing production capacity to help support preparedness efforts.

"BioFire Defense is actively engaging with public health stakeholders and international contacts to monitor the evolving outbreak situation and assess potential support needs," the spokesperson said.

Funding shortages, Ancia said, were having a tremendous impact on the WHO's ability to fight Ebola.

The United States officially left the WHO in January and under President Donald Trump has slashed global health spending, but cooperation with the US on fighting the outbreak was working "very, very well", Ancia said.

"We understand that we cannot receive the funding, it's okay, but we want to keep talking, we want to keep exchanging information, and we want to collaborate," she said.

The UN humanitarian office OCHA said it had only received 34% of $1.4-billion in funding for its appeal for the Democratic Republic of Congo this year, adding that more than half came from Washington.