The United States pledged $60 million on Monday to a new United Nations-backed antiterrorism force from five African countries including Niger, where four American soldiers were killed early this month.

The pledge, announced by Secretary of State Rex W. Tillerson in Washington and Ambassador Nikki R. Haley at the United Nations, came as the United Nations Security Council was meeting about the vast and lawless Sahel region in western and north-central Africa, where the force will operate.

“Defeating terrorism depends on making sure terrorist organizations cannot have safe havens on any continent,” Mr. Tillerson said in a statement announcing the pledge, which will require congressional approval.

“This is a fight we must win, and these funds will play a key role in achieving that mission,” he said.

The 5,000-member force of soldiers and police officers, with recruits from Burkina Faso, Chad, Mali, Mauritania and Niger, was created by those countries in February to combat the growing threat of extremist groups, drug smugglers and human traffickers in the Sahel.

With little government control, the region is regarded as a fertile haven for Al Qaeda, the Islamic State and their offshoots.

The Security Council unanimously endorsed the force in June but questions of how its $500 million annual cost will be financed have not been resolved. The European Union, which has committed about $58 million to the force, will convene a donor conference in December.

The United States, the biggest single financial contributor to United Nations peacekeeping operations, has been seeking to cut their costs and had resisted proposals to commit peacekeeping resources to the five-nation Sahel force.