WASHINGTON, Nov 28 (AFP) - A communications line created between the militaries of the United States and Russia at the start of Moscow's war against Ukraine has been used only once so far, a U.S. official told Reuters.

The official, who spoke on condition of anonymity, said that the United States initiated a call through the "deconfliction" line to communicate its concerns about Russian military operations near critical infrastructure in Ukraine.

Reuters is the first to report on the use of the deconfliction line, beyond regular testing.

Few details are known surrounding the specific incident that led to the call on the line, which connects the U.S. military's European Command and Russia's National Defense Management Center.

The official declined to elaborate but said it was not used when an errant missile landed in NATO-member Poland on Nov. 15, killing two people. The blast was likely caused by a Ukrainian air defense missile but Russia was ultimately responsible because it started the war in late February, NATO said.

Although the U.S. official declined to specify which Russian activity raised the U.S. alarm, there have been publicly acknowledged incidents involving Russian fighting around critical Ukrainian infrastructure.

These include Russian operations around Ukraine's Zaporizhzhia nuclear power plant, Europe's biggest, which is under Russian control.

Ukraine has also voiced concerns Russia might blow up the Nova Kakhovka dam, which holds back an enormous reservoir in southern Ukraine. Bursting the dam would send a wall of water flooding settlements below, including towards the strategic regional capital Kherson, which Ukrainian forces recaptured on Nov. 11.

U.S.-Russia communications have been in the spotlight since the start of Russia's invasion of its neighbor, given the grave risk that a miscalculation by either side could cause a direct conflict between the nuclear-armed nations.