Six U.S. military planes arrived in the Ebola hot zone Thursday with more Marines, as West Africa’s leaders pleaded for the world’s help in dealing with a crisis that one called “a tragedy unforeseen in modern times.”

“Our people are dying,” Sierra Leone President Ernest Bai Koroma lamented by video conference at a World Bank meeting in Washington. He said other countries are not responding fast enough while children are orphaned and infected doctors and nurses are lost to the disease.

Tom Frieden, director of the U.S. Centers for Disease Control and Prevention, said he was reminded of the start of the AIDS epidemic.

“We have to work now so this is not the next AIDS,” Frieden said.

The fleet of planes that landed outside the Liberian capital of Monrovia consisted of four MV-22 Ospreys and two KC-130s. The 100 additional Marines bring to just over 300 the total number of American troops in the country, said Maj. Gen. Darryl A. Williams, the commander leading the U.S. response. Williams joined the American ambassador to Liberia, Deborah Malac, at the airport to greet the aircraft.

As vehicles unloaded boxes of equipment wrapped in green-and-black cloth, the Marines formed a line on the tarmac and had their temperatures checked by Liberian health workers.