U.S. recognition of Russia's takeover of Crimea is not a question of time, Vice President Joe Biden assured Ukrainian leaders on Friday, the Los Angeles Times reported.

"The United States will never recognize (the) annexation, and (Biden) warned Moscow that it faces greater isolation if it continues its 'provocative action,'" the newspaper detailed.

The vice president was in Kiev on the first anniversary of the beginning of massive protests that earlier this year forced President Viktor Yanukovich from power. "Ukraine has since been torn apart by a pro-Russia rebellion in the east of the country and the loss of the Crimean peninsula, which Russia seized days after its loyal ally Yanukovich was forced to flee," the Los Angeles Times noted.

During his two-day visit, Biden met with Ukrainian President Petro Poroshenko and Prime Minister Arseniy Yatsenyuk. During a joint news conference with Poroshenko on Friday, he "denounced Russian behavior in eastern Ukraine as 'a flagrant violation' of the international system" and "called on Russia to withdraw its troops and armaments," the Washington Post noted.

But Biden also stressed on what Kiev needed to move forward, such as cleaning up "its governance, economy and judiciary, and to end endemic corruption," the newspaper added.

The vice president said that once those steps are taken, "international partners" who have been wary of giving their financial assistance will be more open to the idea. 

The vice president engaged in a discussion with his hosts on Ukraine's long-standing request for lethal military equipment, telling them the United States had not yet made decisions on the matter, according to the Washington Post. The United States has provided the country with about $118 million worth of non-lethal military aid, the newspaper added.

On Thursday, Russia warned the United States not to supply weapons to Ukrainian forces fighting pro-Russian separatists in eastern Ukraine, according to the English-language Moscow Times. A spokesman for the Russian Foreign Ministry said such a move would send a "very serious signal."

"That (would be) a direct violation of agreements reached, including (agreements reached) with the participation of the United States," spokesman Alexander Lukashevich told the newspaper.