Tedros, however, cited an attack on an Ebola treatment centre in Ituri province in which two people died as a sign of ongoing difficulties in tackling the spread of the disease.
"Despite all this progress, we continue to face significant challenges, including mistrust and violence," the WHO director-general said.
There are currently no approved vaccines or treatments for Ebola's Bundibugyo strain, which has caused more than 1400 cases in DR Congo, including 438 deaths.
DR Congo has recorded an average of 38 new confirmed cases a day over the past two weeks, Tedros said.
The trial, which could take months to complete and includes more than 1000 patients, will evaluate Mapp Biopharmaceutical's experimental MBP134 antibody as a standalone treatment for Bundibugyo Ebola, as well as in combination with Gilead Sciences' antiviral drug remdesivir, the WHO said.
There are enough drug supplies for the trials, according to the WHO, which said it was in discussions with the United States, which donated supplies of MBP134, and Gilead to ensure patients would be able to access the drugs after the trials if they are shown to be safe and effective.
Gilead said on Thursday it has donated more than 2000 vials of remdesivir for the trial, in addition to the 2000 vials it provided for emergency use in June.
The WHO chief also said other parts of the response were showing improvement: there are 10 laboratories that can test for Ebola and follow-ups are being conducted for four in five contacts, although more contacts still needed to be identified per case.
Treatment capacity has also expanded, with 650 beds available, of which about 96 per cent are currently occupied.
Tedros said the WHO and its partners were working to add 300 additional beds.
Meanwhile, the race to produce a rapid antigen test to diagnose the Bundibugyo virus has closed in on five possible manufacturers that could begin field trials in eastern DR Congo within weeks, a global alliance for diagnostics leading the effort told Reuters.
A rapid test would let health workers get results on the spot, which would help them identify and isolate cases and better contain the ongoing outbreak.
There is no rapid test currently approved for the Bundibugyo virus, and officials often have to wait days for laboratory test results.
The five potential test manufacturers - selected from 21 candidates - include two in west Africa, one in the US and two in South Korea, according to FIND, the Geneva-based nonprofit that works on developing diagnostics.
In previous Ebola outbreaks, rapid tests have only been used to test dead bodies.
But Kavi Ramjeet, head of business development at FIND, said the goal is to develop rapid tests that could be used on blood samples of live patients.
Ramjeet said the first tests could reach the field in mid-July, and that manufacturers had been selected based on criteria that included the ability to quickly roll out thousands of tests.
The timing for the rollout would depend on progress getting approval, which was too early to define, he said.
