WASHINGTON -- U.S. Defense Secretary Jim Mattis rebuffed suggestions that private military forces could join the fight in Afghanistan, and possibly replace regular U.S. troops.

Mattis made the comments at a Pentagon press briefing August 28, where he was asked about such proposals that have been floated in Washington, D.C. and elsewhere.

"When Americans put their nation's credibility on the line, privatizing it is probably not a wise idea," Mattis said.

The main proponent of the idea has been Erik Prince, the former head of a controversial private military contractor once known as Blackwater.

A longtime Republican supporter and brother of President Donald Trump's education secretary, Prince has argued that using private mercenaries would help decrease the US military presence in Afghanistan after 17 years of war.

In recent weeks, Prince has engaged in a public relations campaign, including publishing online videos, to press his arguments.

About 14,000 U.S. troops are in Afghanistan, providing the largest contingent of NATO's mission and helping lead increasingly tense offensive operations against Taliban fighters.