NATO Secretary-General Jens Stoltenberg has affirmed the alliance's "unwavering support" for Ukraine's territorial integrity and has called on Russia to remove its "thousands of soldiers from Ukraine and stop supporting the militants with command-and-control and military equipment."
Stoltenberg made the comments in Kyiv after talks with President Petro Poroshenko and other officials on July 10.
"I am convinced that an investment in Ukraine's security is an investment into the security of NATO and its member states that pays off," Stoltenberg said, adding that the alliance is "learning a lot" from the cyberattacks that Ukraine has endured in recent months.
He called for the complete implementation of the Minsk agreements, aimed at ending the conflict in eastern Ukraine that has left more than 10,000 people dead since early 2014.
Stoltenberg also reaffirmed that NATO members do not accept Russia's 2014 annexation of the Ukrainian region of Crimea.
Russia has denied military involvement in the conflict in Ukraine despite substantial evidence it has provided troops, mercenaries, and military equipment via the part of Ukraine's border that is controlled by the separatists.
Asked about Stoltenberg's remarks, Kremlin spokesman Dmitry Peskov told journalists in Moscow that "Russia has never had and has no servicemen in Ukraine." Peskov also said that Ukraine's possible entry into NATO would "not help strengthen stability and security in Europe."
Stoltenberg added that monitors from the Organization for Security and Cooperation in Europe must be allowed unfettered access to all of Ukraine's territory.
Poroshenko and Stoltenberg announced that the alliance would "begin a discussion" with Ukraine on an action plan for the country's possible eventual NATO membership.
Poroshenko pledged to implement reforms aimed at meeting NATO standards by 2020.
Poroshenko said "Ukraine has clearly defined its political future and its future in the sphere of security."
Last month, Ukraine's parliament adopted a law establishing NATO membership as a priority foreign-policy goal.
Ukrainian Deputy Prime Minister Ivanna Klympush-Tsyntsadze said the same day that Ukraine seeks "partnership" with NATO.
"We have something to contribute to this system," she said.
Stoltenberg's visit comes one day after U.S. Secretary of State Rex Tillerson visited Kyiv and offered strong support for Ukraine, stressing that Russia must take the necessary steps to end the conflict in eastern Ukraine.