WASHINGTON — Liberia is making progress against Ebola but stamping out the final cases so the epidemic doesn't flare up again will be difficult, President Ellen Johnson Sirleaf told U.S. lawmakers Wednesday.

In an unusual address by video to a Senate subcommittee, Sirleaf said about 10 new cases a day have been confirmed in Liberia over the past week compared with around 100 a day at the epidemic's peak.

She thanked the U.S. for helping to fight Ebola, saying it had "awakened the world to the scope and magnitude" of the epidemic in West Africa.

But she made clear that much work remains, saying Liberia was "now in the most critical stage of response," when complacency could allow resurgence.

"Traveling that last mile to zero new cases will be much more difficult because the disease has retreated and must now be chased down in every corner," Sirleaf told the Senate Foreign Relations African affairs subcommittee.

Sirleaf said Liberia also needs help strengthening its health system to prevent future outbreaks. She noted that Liberia has only one doctor for every 100,000 people.

This week, Congress was preparing to vote on a spending bill that includes $5.4 billion in additional funds to fight Ebola abroad and strengthen preparedness at home. President Barack Obama had requested nearly $6.2 billion; left out of the spending bill was a contingency fund for unanticipated developments.

Part of the new money would go to improve the health infrastructure in Liberia, Guinea and Sierra Leone — the countries hardest-hit by Ebola — so they can spot disease outbreaks early, and better contain them. The epidemic devastated the countries' already weak health systems, with many clinics closing and people unable to find care for malaria and other common illnesses.

The new Ebola funding also would go to shoring up disease detection in other high-risk countries, to guard against the next contagion.

Ebola has infected more than 17,000 people in West Africa, and killed more than 6,000. Authorities say infection rates in Guinea also appear to be stabilizing. But cases are escalating in neighboring Sierra Leone, which imposed a two-week "lockdown" in one part of that country Wednesday in attempt to stem the spread.