Congo has been battling an outbreak of a rare type of Ebola since May, with no approved treatment or vaccine.
The Africa Centres for Disease Control says it is the fastest-growing Ebola outbreak on the continent.
"Perhaps the most alarming finding is that many of the newly reported deaths are people who died in their communities without ever reaching a health facility and without receiving care," the World Health Organization's Chikwe Ihekweazu said after returning from Bunia, in Ituri province, one of the worst-hit cities.
"And as of today, 80 per cent of new cases are outside our contact lists and so are coming to us from unknown chains of transmission."
People who die outside the health system cannot be isolated, treated or have their contacts traced promptly, increasing the risk of further transmission.
The outbreak, Ihekweazu said, "continues to outpace the response efforts".
As of Monday, at least 1926 people have been infected, of whom 702 have died, in three provinces in Congo from the rare Bundibugyo virus, Congolese authorities said.
Cases have also been confirmed in neighbouring Uganda.
Ihekweazu told reporters in Geneva that his visit to Bunia had been "quite encouraging on many fronts, but also deeply concerning".
Treatment capacity in Bunia is now close to 800 beds, with capacity increasing every week, and lab capacity has grown from one- to-14 labs, an effort the emergency chief lauded.
However, Ihekweazu said that despite "our best efforts ... we have not caught up in the race".
A funding gap, attacks on health centres, an ongoing conflict in eastern Congo, and mistrust among local communities have hampered the response.
Dozens of healthcare workers at an Ebola virus treatment centre in northeast Congo went on strike over unpaid salaries and bonuses on Monday.
On Tuesday, they agreed to resume work under the condition that the government pays them within 72 hours.
"Just one day of strike action has already caused damage. Patients were unable to access the centre," the striking health workers said in a statement.
"We hold the government solely responsible for any loss of life if the site closes after this ultimatum."
The Congolese authorities declared a fresh Ebola outbreak on May 15 after the disease had been transmitting for weeks without official detection, according to the WHO.
Clinical trials for treatment began last week.
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