WASHINGTON (Reuters) - An International Criminal Court investigation of possible war crimes by U.S. forces in Afghanistan is not "warranted or appropriate," the U.S. State Department said on Tuesday after prosecutors in The Hague found initial grounds for such a probe.

State Department spokeswoman Elizabeth Trudeau said the United States was not a party to the Rome Statute that created the International Criminal Court in The Hague and had not consented to ICC jurisdiction. She also said Washington had a robust justice system able to deal with such complaints.

"The United States is deeply committed to complying with the law of war," Trudeau told reporters at a news briefing. "We do not believe that an ICC examination or investigation with respect to actions of U.S. personnel in relation to the situation in Afghanistan is warranted or appropriate."

Her comments came a day after prosecutors at the International Criminal Court said in a report that there was a "reasonable basis to believe" that U.S. forces had tortured at least 61 prisoners in Afghanistan and another 27 at CIA detention facilities elsewhere in 2003 and 2004.

The prosecutors' office, headed by Prosecutor Fatou Bensouda, said it would decide imminently whether to pursue a full investigation. The results could lead to charges being brought against individuals and the issuing of arrest warrants.

The United States occupied Afghanistan in 2001 as it went after al Qaeda leaders behind the Sept. 11 attacks on New York and Washington. Crimes also may have been committed at U.S. Central Intelligence Agency facilities in Poland, Lithuania and Romania, where some people captured in Afghanistan were taken, prosecutors said.

The U.S. Justice Department between 2009 and 2012 investigated CIA mistreatment of detainees, including a full criminal investigation into two deaths in U.S. custody, but ultimately decided against prosecuting anyone.

Some U.S. military personnel have been prosecuted for murder and other crimes in Afghanistan.

The ICC was established in 1998 to prosecute war crimes and crimes against humanity. Trudeau noted the United States has "engaged with the ICC and we've supported ICC investigations and prosecution of cases that we believe advance our values in accordance with U.S. law."

But she said the U.S. military was held to "the highest possible standards" and the United States had systems of accountability capable of dealing with war crimes allegations.

"We do an extraordinary job of investigating ... credible allegations, holding ourselves accountable, holding our personnel accountable and closing investigations in a manner that serves justice," she said.

(Reporting by David Alexander and Thomas Escritt; Editing by Tom Brown)