According to the latest figures, the virus is "growing exponentially," Julien Harneis told reporters in Geneva from Bunia, at the epicentre of the outbreak.
He said that in the last three months, "more than 2,500 people" had died, "and half of those in the last 20 days".
"The epidemic is spreading widely. It's now covering an area that is bigger than France.... It is growing faster and wider than the Ebola response."
The DRC's 17th Ebola outbreak is thought to have already been spreading for several weeks before it was officially declared on May 15.
It has hit regions in the north and east where the presence of the state is weak, health infrastructure is lacking and myriad armed groups have roamed for decades.
- 'Alarming' -
The Doctors Without Borders (MSF) charity also cautioned Friday that the outbreak was spreading "at an alarming rate".
"This epidemic continues to spread, moving faster than the response can keep up," MSF president Javid Abdelmoneim said in a statement.
Harneis stressed the need to ramp up the response, which has faced criticism for its slowness and poor organisation, and is also hampered by mistrust among the population.
Cases have so far been reported in six DRC provinces, including near the border with South Sudan, as well as in neighbouring Uganda.
Uganda, which saw two deaths from 20 Ebola cases, declared itself free of the virus last month.
There is no approved vaccine or specific treatment for the Bundibugyo virus strain behind the current surge, although a number of trials are underway.
And the World Health Organization said Thursday that the DRC would receive 70,000 doses of Ervebo vaccine, which has proved effective against the more common Zaire strain.
Earlier this month, WHO vaccine experts recommended a full-scale human trial of Ervebo, the only existing approved Ebola vaccine, to see if it offered cross-protection against the Bundibugyo strain.
Early data, notably from animal trials, has showed it may give some protection against Bundibugyo, but "it is not yet known whether Ervebo is protective against the Bundibugyo virus in humans", WHO spokesman Christian Lindmeier stressed Friday.
- 'Brutal' -
Reining in the virus, which spreads by contact with bodily fluids and causes a haemorrhagic fever, has faced a multitude of other challenges.
Harneis highlighted the rampant insecurity and weak health system in eastern DRC.
"The law and order is undermined, and the basic services, particularly health, have been fragmented," he said, describing the operating conditions as "brutal".
Some 160 healthcare workers have fallen ill with Ebola since the start of the outbreak, including 43 who have died from the virus, he said, also pointing to repeated attacks on ambulance and other frontline workers.
Harneis also highlighted the barely functioning banking system, which has made it difficult get wages to those working on the outbreak, even as he said international funding cuts had reduced the capacity of humanitarian organisation by a third.
He called for more international support.
"If we provide more staffing, if we get more resources into remote areas across ... the east of the Democratic Republic of Congo, within months we can slow the transmission and move to stopping it," Harneis said.
"If we do not, ... then this epidemic will become more deadly. It will spread wider and it risks to spread into neighbouring countries."
