UNITED NATIONS (AP) — An informal U.N. Security Council meeting to mark the 20th anniversary of the statute that created the International Criminal Court was boycotted Friday by the United States, which is not a party to the world's first permanent war crimes tribunal.

There is no requirement that the 15 council members attend informal meetings, but it is rare for a member to boycott, especially a veto-wielding permanent member like the United States. Russia and China, also permanent members that don't support the ICC, attended the session along with other opponents and spoke.

A U.S. official told The Associated Press that the Trump administration decided not to participate "after careful consideration."

"We recently noted concerns about any potential ICC investigation of U.S. personnel related to the situation in Afghanistan," said the official, who was not authorized to speak publicly. "Other aspects of our policy are under review."

In November, ICC prosecutor Fatou Bensouda sought judicial authorization to begin an investigation of possible war crimes in Afghanistan.

At the time, she said evidence existed of war crimes committed "by members of the United States armed forces on the territory of Afghanistan, and by members of the U.S. Central Intelligence Agency (CIA) in secret detention facilities in Afghanistan" operated mostly between 2003 and 2004 as well as in countries that signed the Rome Statute that established the ICC.

The Rome Statute was adopted on July 17, 1998, but the ICC wasn't officially established until July 1, 2002, with a mandate to prosecute war crimes, crimes against humanity and genocide.

U.S. President Bill Clinton signed the Rome Statute, but his successor, George W. Bush, renounced the signature, citing fears that Americans would be unfairly prosecuted for political reasons. While the U.S. is not a member state, its citizens can be charged with crimes committed in countries that are members — and Afghanistan has been a member since May 2003.

The Netherlands's U.N. ambassador, Karel Van Oosterom, whose country hosts the ICC and who presided at Friday's meeting, stressed the court's importance for prosecuting individuals accused of the world's worst atrocities but also its role in preventing war crimes and protecting civilians.

"What is most important is we will continue to fight and uphold the values of the International Criminal Court," he said.

As for the U.S. boycott, Van Oosterom said, "it's an informal meeting of the Security Council, so it's up to each and every member of the council whether to attend."

Bensouda also addressed the informal council meeting, but did not mention the Afghan case. She focused on the importance of the relationship between the ICC and the Security Council, which can refer cases to the court. It has done so twice — for the conflict in Sudan's western Darfur region and events in Libya in 2011 that led to the overthrow and death of longtime ruler Moammar Gadhafi.

Bensouda sidestepped a question about the U.S. boycott, saying that what is important is "the overwhelming support that we have had from various members of the council, but also of the wider (U.N.) membership" for the ICC's work and for making the Rome Statute universal.