The outbreak, already the third-largest on record, persisted for weeks undetected, say health officials, who are now behind the curve and struggling to bring it under control.
WHO Director-General Tedros Adhanom Ghebreyesus called for more international support to stop the disease's spread before he travelled to Congo's Ituri province where the first cases were confirmed.
There he said he saw some encouraging signs - including five certified recoveries - as well as the need to ramp up testing and treatment capacity and promote trust in health workers.
"This Ebola can be stopped when the community owns the agenda and with strong government leadership," he said after meeting with President Felix Tshisekedi in Kinshasa.
"We need to strengthen the capacity of the health systems in the affected areas."
Tedros was expected to return to Geneva after meeting Tshisekedi.
In a joint statement, the WHO and the Congolese government acknowledged it was "a challenging time", with health workers struggling to detect and isolate cases, trace contacts and promote safe burials.
The WHO said on Friday there were 906 suspected cases of Ebola in Congo, including 223 suspected deaths under investigation. And Congo's government said late Sunday the number of confirmed cases had increased to 282, with 42 deaths, after 19 new positive test results were recorded.
There have been 264 confirmed cases in Ituri province as well as 15 in North Kivu province and three in South Kivu province, according to data distributed by the communications ministry.
All three provinces have been hit by armed conflict that has caused mass displacement, further complicating the Ebola response.
An attack at the weekend by fighters with the Allied Democratic Forces, a rebel group linked to Islamic State, killed 15 civilians and a soldier in the city of Beni in North Kivu, a government statement said.
Ebola cases have also been confirmed in neighbouring Uganda.
The International Rescue Committee, however, warned the outbreak was likely significantly larger and more advanced than official figures suggest.
The aid agency said the virus may have been spreading for up to three months before the first official cases were detected in mid-May. And with only 20 per cent of contacts currently being traced, it said, health authorities are struggling to identify and isolate new chains of transmission.
"When four out of five contacts are not being traced, it becomes incredibly difficult to contain the outbreak or even understand its true scale," said Rachel Howard, IRC's senior technical emergency health adviser.
While Congolese officials are well-versed in fighting Ebola, they have little experience with the Bundibugyo strain of the virus, which is responsible for the current outbreak and for which there is no approved vaccine.
Global health organisation CEPI will give roughly $US60 million to Moderna and two other groups to accelerate the development of shots against Ebola Bundibugyo.
China said on Monday it would send a team of medical specialists to Congo to assist with the outbreak.
