NATO's military commander accused Russia on Wednesday of making irresponsible nuclear threats, which he said were intended to make NATO think twice about how it responded to Russia's actions in Ukraine.
US Air Force General Philip Breedlove, NATO's supreme allied commander in Europe, also voiced concern at an increase in fighting in several areas of eastern Ukraine despite a declared ceasefire.
He said he saw a pattern developing similar to previous occasions when pro-Russian separatists took advantage of ceasefires to build their strength before launching a new offensive.
Breedlove stressed, however, that he could not judge the intentions of "the Russians that are leading these forces".
Russia denies providing any troops or arms to support rebellion in eastern Ukraine and accuses Kiev of violating the current ceasefire.
Russian officials have repeatedly made references to Moscow's nuclear arsenal as tensions with the West rose over Russia's annexation of Ukraine's Crimea region last year and what NATO says is Russian military support for the separatists.
President Vladimir Putin said in March that Moscow had been ready to put its nuclear forces on alert to ensure the annexation of Crimea, while Russia's ambassador to Denmark said Russian nuclear missiles could be aimed at Danish navy ships if Copenhagen joined NATO's missile defense system.
A Russian Foreign Ministry official also said in March that Moscow had the right to deploy nuclear arms in Crimea, though he knew of no plans to do so.
"This is not responsible language from a nuclear nation," Breedlove told reporters during a meeting of NATO foreign ministers in Turkey.
"I do not think that it aids in any serious negotiations or any serious deceleration of the problem (in Ukraine) and so I believe that all the nuclear nations should react responsibly in not only their actions but their words when it comes to nukes.
"I do believe that those remarks were put out ... to send a message ... It was put out there to intend to give us pause in our decision-making," Breedlove said when asked if he believed the comments were intended to deter NATO from responding with conventional weapons to Russia's actions in Ukraine.
NATO and Ukraine voiced concern in a joint statement at the meeting about Russian statements on the possible future stationing of nuclear weapons in Crimea.
"The messages by the Russian side about the ... possibility of placing nuclear weapons in Crimea is a complete breach of international obligations," Ukrainian Foreign Minister Pavlo Klimkin told a news conference.
On the military situation in Ukraine, Breedlove said the port city of Mariupol and nearby Shyrokyne had both seen increased shelling, fighting and deaths in the last few days. "This is a very fragile ceasefire and we need to return to a ceasefire situation," he said.
"I think that what we see is a pattern that matches previous patterns during ceasefires ... We see training, we see re-equipping, we see new forces, new money, organisation, increased command and control, all these things are in a pattern across the past several times where we have seen a push (offensive)."