"Under international law, the United States has the right to respond to Russia's breaches of the New START Treaty by taking proportionate and reversable countermeasures in order to induce Russia to return to compliance with its obligations," a spokesperson for the National Security Council said.

"That means that because Russia's claimed suspension of the New START Treaty is legally invalid, the U.S. is legally permitted to withhold our biannual data update in response to Russia's breaches," the spokesperson added.

(Reporting by Steve Holland; Writing by Jasper Ward; Editing by Rami Ayyub)