The United States fears that recent Ukrainian drone strikes targeting Russian nuclear early-warning systems could dangerously unsettle Moscow at a time when the Biden administration is weighing whether to lift restrictions on Ukraine using U.S.-supplied weapons in cross-border attacks.

"The United States is concerned about Ukraine's recent strikes against Russian ballistic missile early-warning sites," said a U.S. official, who spoke on the condition of anonymity because of the matter's sensitivity.

Washington has conveyed its concerns to Kyiv about two attempted attacks over the last week against radar stations that provide conventional air defense as well as warning of nuclear launches by the West. At least one strike in Armavir, in Russia's southeastern Krasnodar region, appeared to have caused some damage.