A direct line of communication established between the US and Russian armies at the start of the war in Ukraine has so far been used only once, a US official told Reuters, citing News.ro.

The official, who spoke on condition of anonymity, said the United States initiated the call through a “conflict de-escalation” line to inform the Russian side of its concerns about Russian military operations near critical infrastructure in Ukraine.

Little is known about the specific incident that led to the call to the hotline connecting the US Army European Command and Russia’s National Defense Command Center, Reuters notes.

A U.S. official declined to provide details but said it was not used when a stray missile landed in Poland on Nov. 15, killing two people. The explosion was probably caused by a Ukrainian anti-aircraft missile, although the ultimate responsibility for the start of the war lies with Russia, NATO said.

Although the US official declined to specify which Russian activities have caused concern in the US, there have been publicly acknowledged incidents involving Russian attacks on critical Ukrainian infrastructure. Among them are Russian operations around the largest Ukrainian Zaporizhzhya nuclear power plant in Europe, which is under Russian control.

Ukraine has also expressed concern that Russia could blow up the Nova Kakhovka Dam, a huge reservoir in southern Ukraine, whose failure would flood downstream communities, including the strategic regional capital Kherson, which Ukrainian forces recaptured on November 11 after a Russian retreat.

How Americans communicate with Russians

Direct communication between the US and Russia has been in focus since the beginning of the Russian invasion of the neighboring country, given the serious risk that a miscalculation by one side could lead to direct conflict between the two nuclear powers.

When it was announced in March, the Pentagon said that this direct line between the US and Russian militaries was created to avoid any inadvertent collisions in NATO airspace or on the ground. The Americans also thought it could be useful if the United States needed to evacuate Americans from Russian-occupied territory in Ukraine. One official suggested it could also be used if a Russian fighter jet was following a Ukrainian jet into Polish airspace or if a Russian missile crossed NATO airspace.

The de-escalation line is checked twice a day, and calls are made in Russian, a US official told Reuters. The Russian-speaking representative of the US European Command is initiating those calls from Wiesbaden, Germany, the official said.

The “deconfliction” line, as this direct line of communication is called, is one of the few avenues left by the US and Russian militaries to maintain direct dialogue, Reuters noted.

Other military channels include rare high-level talks between US Defense Secretary Lloyd Austin and Russian Defense Minister Sergei Shoigu.

Top American and Russian generals, US Army Joint Chiefs of Staff Mark Milley and Russian General Valery Gerasimov, have also spoken twice since the start of the war.

White House National Security Adviser Jake Sullivan and CIA Director Bill Burns also had contacts with Russian counterparts.

However, relations between the US and Russia are at their lowest level since the Cold War, and the US State Department said on Monday that Moscow had postponed talks in Cairo aimed at restoring nuclear weapons inspections. The Russian Foreign Ministry confirmed that the negotiations have been postponed. Neither side gave a reason, News.ro cites

During the Cold War, the United States and the Soviet Union maintained such direct lines to varying degrees, Reuters reminds.