"I'm talking about bordering countries like Ivory Coast, Mali, Senegal, Guinea-Bissau," he told reporters.
Those countries are among 11 nations due to attend WHO-brokered Ebola talks in Ghana next week.
To date, there have been 635 cases of haemorrhagic fever in Guinea, Liberia and Sierra Leone, most confirmed as Ebola.
A total of 399 people have died, 280 of them in Guinea.
The outbreak is the first in west Africa, and the largest since Ebola first emerged in 1976 in what is now the Democratic Republic of Congo.
Ebola is believed to be carried by animals hunted for meat, notably bats.
Formenty said highways appeared to be key to the virus's spread from a forested region between Guinea, Liberia and Sierra Leone.
"Of course there are some controls, but we all know that all these borders are quite porous," he said.But Formenty said draconian measures on travel would not help.
"We are not recommending any travel or trade restrictions to be applied to Guinea, Liberia and Sierra Leone," he underlined.
"If we try to institute measures that are going to be seen as restrictive by the population, we are going in fact to fuel the outbreak," he said.
"We favour dialogue with the affected families, the affected villages, the affected districts, rather than trying to be a sort of sanitary police."
- "Not out of hand" -
Last week, Formenty told AFP that a recent surge in cases was partly because efforts to contain the virus were relaxed too quickly after the outbreak appeared to lose pace in April.
"This failure of the system has created a clandestine chain of transmission that explains why we are where we are today," he said.
Medical charity Doctors Without Borders has warned the outbreak is out of control. But Formenty played that down.
"This is not out of hand," he said. "We have been able to control this outbreak in a number of places. In some other places it's been more difficult."
Ebola can fell victims within days, causing severe fever and muscle pain, vomiting and diarrhoea -- and in some cases, organ failure and unstoppable bleeding.
It is deadly in up to 90 percent of cases, and there is no vaccine.
It spreads via bodily fluids including sweat, meaning you can get sick from simply touching an infected person, and patients have to be isolated.
Funeral rites involving touching corpses also pose a threat. Formenty noted it had been difficult to push that message, because the passing of a loved one was such a sensitive time.
"The only way we will succeed is when the people will understand clearly how dangerous it is for their lives to conduct unsafe burials during an outbreak of Ebola," he said.