"NATO firmly supports Ukraine's sovereignty and territorial integrity," the military alliance chief tweeted on Tuesday after the conversation, in a warning to Moscow.
I called President @ZelenskyyUa to express serious concern about Russia’s military activities in and around Ukraine & ongoing ceasefire violations. #NATO firmly supports #Ukraine’s sovereignty & territorial integrity. We remain committed to our close partnership.— Jens Stoltenberg (@jensstoltenberg) April 6, 2021
Amid reports of Russian troop deployments, concern is growing that the conflict between Russia and Ukraine could escalate again.
Russia annexed the Crimean peninsula seven years ago after a controversial referendum. Parts of Ukraine's Donetsk and Luhansk regions along the Russian border are controlled by rebels supported by Moscow.
As a result of the fighting with government troops, more than 13,000 people have been killed, according to UN estimates.
Despite an agreed ceasefire, there have been repeated deadly clashes in eastern Ukraine in the past few weeks.
Zelensky equates NATO with peace
Late Monday, two people were killed according to Kiev, at least one them a soldier. Since the beginning of this year alone, at least 24 soldiers have died on the Ukrainian government side. Separatists recently spoke of some 23 people killed in their area.
Zelensky said membership of NATO was "the only way for peace."
Granting Ukraine a plan to join the alliance would be a "clear signal" for Moscow, Zelensky said, according to a statement released after the call with Stoltenberg.
Ukraine's armed forces are also continuing to work on adopting Western standards, his office said in a statement.
Russia has repeatedly warned against allowing Ukraine to become a NATO member, saying this would be a threat to its security.
"We very much doubt that this will help Ukraine solve its internal problem. From our point of view, it will aggravate the situation," government spokesman Dmitry Peskov said, according to Russian news agency Interfax.
US, Britain expressed fears
Foreign Minister Sergei Lavrov demanded Paris and Berlin step up as well: "We do not see any dynamic action by our French and German colleagues to bring the Ukrainian side to its senses," he was quoted as saying by the ITAR-TASS news agency during his visit to India.
Earlier, the United States and Britain expressed similar fears of an escalation in the conflict and affirmed their support for Kiev.
US State Department spokesperson Ned Price told journalists in a press briefing there were "credible reports" of Russian troop movements along the borders of Ukraine and occupied Crimea.
US President Joe Biden recently reaffirmed to Zelensky Washington's backing for Kiev, Price noted.
"We have asked Russia for an explanation of these provocations, but most importantly what we have signalled directly with our Ukrainian partners is a message of reassurance," Price told journalists.
Prime Minister Boris Johnson also spoke with Zelensky on Monday, and reaffirmed Britain's "unwavering support for Ukraine’s sovereignty and territorial integrity," according to his spokesperson.
"They agreed to continue working closely with international partners to monitor the situation and ensure there is no further escalation," Johnson's spokesperson added.