BRUSSELS -- The head of NATO has demanded that Russia release Ukrainian sailors and naval vessels it seized in a confrontation at sea, while Russian President Vladimir Putin's spokesman says concerns that Moscow could seek to create a "land corridor" linking Russia to Crimea are "absurd."

NATO Secretary-General Jens Stoltenberg and Kremlin spokesman Dmitry Peskov spoke on December 3 as Russia continued to hold the 24 Ukrainian seamen in Moscow jails on border-violation charges Kyiv rejects.

"Russia recently seized Ukrainian ships and sailors near the Kerch Strait. There is no justification for this use of force. We call for calm and restraint. Russia must release the Ukrainian sailors and ships," Stoltenberg said on the eve of a two-day meeting of NATO foreign ministers who will address that issue, among others.

Russia, he said, "must also allow freedom of navigation and unhindered access to Ukrainian ports in the Sea of Azov. Ukrainian vessels -- military as well as civilian -- have the right to navigate through the Kerch Strait and the Sea of Azov."

Russia has been holding the sailors -- crewmen from three Ukrainian naval craft -- since the confrontation on November 25 in which Russian Coast Guard ships rammed a Ukrainian Navy tugboat and fired on the boats before special forces boarded them.

The clash has added to tension over Crimea, which Russia occupied and took over from Ukraine in March 2014, and raised fears of a flare-up in a simmering war between Kyiv and Russia-backed separatists that has killed more than 10,300 people in eastern Ukraine since April 2014.

The Russia-backed separatists hold parts of the Luhansk and Donetsk regions, including a piece of shoreline that lies between the Russian border and the Ukrainian Sea of Azov port city of Mariupol.

Peskov dismissed concerns that Russia could seek to gain access by land to Crimea by seizing or helping the separatists to seize territory on the Azov shore from Mariupol to the isthmus further west that links the Ukrainian mainland to Crimea.

"It's an absolutely absurd claim. It is another attempt to generate tension. Regretfully, attempts like this will most likely continue as we approach" the Ukrainian presidential election scheduled for March 31, he said.

Peskov claimed that "Russia has never invaded anything or created any corridors anywhere."

But in addition to occupying and taking over Crimea, Kyiv, NATO, and Western governments say there is ample evidence that Russia has given the separatists in eastern Ukraine major military support in the war.

Meanwhile, a lawyer for one of the Ukrainian sailors confirmed on December 3 that all 24 had been charged by Russian authorities with "illegal border crossing."

Nikolai Polozov, who is representing Denys Hrytsenko, said the sailors were formally charged on November 27.

Dzhemil Temishev, an attorney for another of the sailors, said on November 29 that 21 of the detainees were being transferred from Russia-annexed Crimea to the Lefortovo detention center in Moscow.

Russian authorities say three other Ukrainian sailors wounded on November 25 were receiving medical treatment at a different, unspecified detention center.

Russian Human Rights Commissioner Tatyana Moskalkova said on December 3 that the detained Ukrainian sailors had food, "clothes, and all the necessities."

In remarks that appeared aimed to portray the jailing of the sailors as a strictly legal issue and tell the world that they were not being mistreated, Moskalkova said on state-run Rossia-24 TV that rights monitors "have visited the sailors and spoken with every one of them."

She said the sailors "have no complaints about custody conditions or medical aid" and that lawyers had access to their clients. Her comments could not be independently verified.

The incident has raised tensions between Kyiv and Moscow to their highest point since Moscow seized Crimea.

Ukraine imposed martial law for 30 days in 10 regions, including all of those that border Russia or have coastlines on the Black Sea or the Sea of Azov.

Russia and Ukraine blame each other for the altercation. Russia closed the Kerch Strait to all sea traffic during the incident, and Kyiv says Russia has continued to deny passage to Ukrainian commercial traffic in and out of Mariupol and Berdyansk, another port on the Sea of Azov.

Western leaders -- including all members of the G7 leading industrialized states -- have called on Russia to release the Ukrainian crew and return the seized navy ships to Kyiv.

With reporting by Rikard Jozwiak in Brussels, TASS, and Interfax