The US military has requested that the State Department notify Turkey of its possible plans to sail a warship into the Black Sea, three US officials tell CNN, a move they said is a response to Russia's actions against Ukraine in the Kerch Strait, which connects the Black Sea and the Sea of Azov.
For its part, the Russian Ministry of Defense announced Wednesday that its troops in eastern Crimea, which Moscow seized from Ukraine in 2014, conducted a military drill with its Pantsir anti-aircraft missile systems.
The Russian Ministry of Foreign Affairs also claimed Wednesday that the Ukrainian government had undertaken "active preparations" for a military offensive in eastern Ukraine, using the recent declaration of martial law in select regions as a cover.
"Substantial offensive components are being transferred to the region, which are being dispersed along the entire contact line," spokesperson Maria Zakharova said Wednesday at a press briefing.
The US is required to notify Turkey about the passage through the straits under the Montreux Convention, a 1936 treaty that governs the passage of military vessels through the Bosporus and the Dardanelles, which connect the Mediterranean to the Black Sea.
Under the rules of the treaty, countries that do not have a coastline on the Black Sea are required to provide Turkey with at least 15 days' notice prior to transiting the straits.
A State Department spokesman said, "the United States carries out its activities consistent with the terms of the Montreux Convention. We will not, however, comment on the nature of our diplomatic correspondence with the Government of Turkey."
Two of the officials said that the notification was to provide the Navy the option to move a warship into the area, but that this option would not necessarily be selected.
"Our US 6th Fleet is always prepared to respond where called," said Cdr. Kyle Raines, a spokesman for the Fleet, which oversees naval operations in the region.
"We routinely conduct operations to advance security and stability throughout the US 6th Fleet area of operations to include the international waters and airspace of the Black Sea," Raines told CNN. "We reserve the right to operate freely in accordance with international laws and norms," he added.
The planning comes amid heightened tensions between Washington and Moscow on a range of issues.
The US warship conducted what the Navy refers to as a "Freedom of Navigation Operation" to push back against Russian claims on the waters. A spokesman told CNN Wednesday that Moscow lays claim to areas that far exceed the 12 miles from the Russian coastline that is guaranteed by international law.
The USS "McCampbell sailed in the vicinity of Peter the Great Bay to challenge Russia's excessive maritime claims and uphold the rights, freedoms, and lawful uses of the sea enjoyed by the United States and other Nations," US Navy Lt. Rachel McMarr, a spokesperson for the US Pacific Fleet, told CNN in a statement.
Russia engaged in a confrontation on Nov. 25 with Ukrainian vessels around the Kerch Strait, which links the Azov Sea and the Black Sea. Russia rammed and fired on Ukrainian naval vessels, subsequently capturing three ships and detaining 24 service members. Ukraine imposed martial law for 30 days in several regions around the country in response, and has barred entry to male Russian nationals aged 16 to 60 for the duration of the state of emergency.
Russia's actions and its detention of Ukrainian sailors prompted President Donald Trump to cancel a planned meeting with Russian President Vladimir Putin during the G-20 in Argentina.
"Based on the fact that the ships and sailors have not been returned to Ukraine from Russia, I have decided it would be best for all parties concerned to cancel my previously scheduled meeting in Argentina with President Vladimir Putin," Trump tweeted from Air Force One en route to Argentina.
Ukraine says Russia had blocked traffic around the Kerch Strait, but Russia says the passage has been operating as usual, except for weather-related delays.
Secretary of State Mike Pompeo, speaking at a NATO conference in Brussels Tuesday, said the alliance "will collectively develop set of responses that demonstrate to Russians this behavior is unacceptable."
Russia has "partially unblocked" Ukraine's ports on the Azov Sea, allowing ship traffic though the Kerch Strait, Ukrainian Minister of Infrastructure Volodymyr Omelyan said Tuesday in a statement posted on the ministry's website.
The last US ship to enter the Black Sea was the fast transport ship USNS Carson City in October. The USS Carney, an Arleigh Burke-class guided-missile destroyer, left the Black Sea at the end of August.
Under the rules of the Montreux Convention, US ships can only be in the Black Sea for 21 days.
CNN's Elise Labott, Jennifer Hansler and Nathan Hodge contributed to this report