WHO director-general Margaret Chan appealed Wednesday for continued U.S. aid to fight Ebola in West Africa and said that pharmaceutical companies have a responsibility to protect health workers.

Chan spoke as major pharmaceutical companies reaffirmed Wednesday that they will not produce Ebola vaccines or drugs. The companies insist that such a move could endanger health workers now working to contain Ebola in Liberia, Sierra Leone and Guinea.

Dr. Chan, who visited Liberia and Sierra Leone earlier this month, said the only way to bring an end to the Ebola outbreak and save lives is to find a way to fully isolate each patient and quarantine anyone who comes into contact with them. That task would be complicated by the lack of available vaccines or drugs, and Chan admitted there is little sign that any other approach can succeed.

Her words appeared to appeal to the Obama administration, which has held discussions with international pharmaceutical companies about Ebola vaccines, which are still in development. Although U.S. officials have said they would release an experimental Ebola vaccine to doctors in Liberia and Nigeria if that helped expedite the development process, pharmaceutical companies have said they need to see better results before releasing the vaccine into the market.

In an interview with The New York Times, Chan cautioned against “over-constrained” American funding, saying it could unfairly punish health workers and pay the price for having put itself first. She urged the Obama administration to move quickly to get all the aid to West Africa that it promised before curtailing its assistance “because in the meantime, there is already a lot of damage done to people and their families.”

“For me, the issue is not, do we take this over and then that will be the end of it?” Chan said. “It is, for me, an asset to know that there is one more intervention and one more response that is, should I say, more effective. So that we can go from there to what is the best to respond to it after.”

In Washington, National Security Adviser Susan Rice said the administration is doing its “assessment” of how to proceed on Ebola.

“My sense is, from conversations with the Secretary of State, (President Barack) Obama, as well as the president of the World Health Organization, all of us recognize this is an issue the world needs to take very seriously,” Rice said. “And I think from that standpoint we are putting a very high priority on it and will continue to do so.”

Rice said she was concerned that the language the pharmaceutical companies used had a tone of “containment.”

“We don’t want to get stuck in the notion that if we do a vaccine, people die,” Rice said. “We understand people are sick, that there’s so much that needs to be done.”