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 added that monitors from the Organization for Security and Cooperation in Europe must be allowed unfettered access to all of Ukraine's territory, including the conflict zone in the east.
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.
Poroshenko, speaking to reporters alongside Stoltenberg, said Ukraine will begin discussions with NATO on an action plan for eventual membership in the 29-member alliance.
"Today we clearly stated that we would begin a discussion about a membership action plan and our proposals for such a discussion were accepted with pleasure," Poroshenko said.
In separate comments issued by his office, Poroshenko said Ukraine was determined to conduct reforms in order to "have a clear schedule of what must be done by 2020 to meet the NATO membership criteria."
Stoltenberg said that Ukraine has "the right to choose its own security arrangements, and added that "NATO's doors remain open."
Last month, Ukraine's parliament adopted a law establishing NATO membership as a priority foreign-policy goal.
But the conflict in the east poses a big obstacle to membership since aspiring members must settle international disputes by peaceful means before joining the alliance.
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'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.
With reporting by AP, Reuters, and Interfax