MONROVIA, Liberia -- United Nations Secretary-General Ban Ki-moon praised health workers battling Ebola in Sierra Leone and Liberia on Friday, saying they have shown "the most noble face of humankind" amid an epidemic that has killed more than 6,900 people in West Africa.

Ban, who made stops in both countries on Friday, travels Saturday to Guinea where the Ebola virus first emerged a year ago.

"Today we have reason to be cautiously optimistic that this terrible outbreak can be defeated," said Ban at a news conference with Liberia's president.

"Our response strategy is working -- where people are gaining access to treatment, where contacts are being traced, burials are becoming safer, communities are mobilizing to protect themselves," he said.

Nearly 3,300 of the Ebola deaths have been in Liberia, where health officials say the spread of the virus has shown signs of slowing in recent weeks. Concern has grown about the situation in neighbouring Sierra Leone, where the Ebola is now spreading the fastest in the region.

During his stop in Freetown, Sierra Leone, on Friday afternoon, Ban visited an Ebola treatment facility where he praised the health workers and promised continued support from the international community.

"The past months have been filled with sorrow," he said. "The virus has eaten away at the very fabric of society -- how people live, how they love, how they die."

The U.N. chief's visit to Liberia comes on the eve of crucial midterm senatorial elections. Some 139 candidates are vying for 15 seats in the senate, including former soccer star George Weah and Robert Sirleaf, the son of Liberia's president.

Health authorities have deployed 4,700 thermometers to voting places around the country to take voters' temperatures as people go to the polls. Authorities also have deployed 10,000 bottles of sanitizers to polling places amid concerns about Ebola.

The virus is transmitted through direct contact with the bodily fluids of sick people, and one of the main symptoms is a high fever. Some critics have questioned whether the polls can be credible at a time the country is battling such a deadly epidemic.

- Associated Press writer Clarence Roy-Macaulay in Freetown, Sierra Leone, contributed to this report.