GENEVA (Reuters) - United Nations human rights experts called on the United States on Friday to stop its “threats” and visa bans against the International Criminal Court (ICC), which they rejected as “improper interference”.

U.S. Secretary of State Mike Pompeo said a week ago that the United States will withdraw or deny visas to any ICC personnel investigating possible war crimes by U.S. forces or allies in Afghanistan. The visa restrictions may also be used to deter ICC efforts to pursue allied personnel, including Israelis, he said.

The Hague-based court has responded that it was an independent and impartial institution and would continue to do its work “undeterred” by Washington’s actions.

In a joint statement, U.N. experts rejected the warnings of measures by Pompeo and National Security Adviser John Bolton last September.

“These threats constitute improper interference with the independence of the ICC and could hinder the ability of ICC judges, prosecutors, and staff to carry out their professional duties,” the statement said.

“We are particularly concerned in light of recent reports of senior ICC staff resigning from their positions as a consequence of these threats,” it said.

The U.S. mission in Geneva had no immediate comment on the statement.

The United States did not ratify the Rome treaty that established the ICC in 2002. U.S. President Barack Obama took some steps to cooperate with it.

The U.N. experts, Diego Garcia-Sayan, special rapporteur on the independence of judges and lawyers, and Michael Forst, special rapporteur on human rights defenders, said that were in touch with U.S. authorities on the issues.

Activist groups, including the American Civil Liberties Union (ACLU), welcomed the experts’ “condemnation”.

“The purpose of the visa restrictions is to block and deter legitimate criminal investigation into serious crimes under international law,” they said.

“Not only might they have a chilling effect on ICC personnel and others advocating for accountability, but they will set a dangerous precedent with serious implications on the overall fight for impunity,” said the ACLU, International Commission of Jurists and International Service for Human Rights.