"This is a truly breathtaking action by an unaccountable political institution masquerading as a legal body," Pompeo told a news conference following the ruling at the International Criminal Court in The Hague.

"All the more reckless for this ruling to come just days after the United States signed a historic peace deal on Afghanistan, which is the best chance for peace in a generation."

Pre-trial judges at the ICC had last year rejected a demand by its chief prosecutor to open a full-blown probe into alleged war crimes committed in the war-torn nation.

Prosecutors at The Hague appealed the move, saying the judges had erred by saying the request was "not in the interest of justice," and the appeals judges agreed with the prosecution.

The US government bitterly opposes the probe.

"We're going to take all the appropriate actions to ensure that American citizens are not hauled before this political body to settle a political vendetta," Pompeo said, with an announcement to be made in the coming weeks.

ICC prosecutors previously said their investigation would include alleged war crimes by US Central Intelligence Agency operatives at detention facilities, referred to as "black sites" in ICC member countries like Lithuania, Poland and Romania.

At least 24 suspects were subjected to torture at these secret prisons between 2003-2004, the prosecutors said.

The ICC move, which overturned a previous court ruling, was hailed by human rights organizations as a "pivotal moment" for victims of the central Asian country's 18-year-war since the 2001 US invasion.