According to its latest ongoing security assessment -- shared with CNN by Ukrainian security sources -- the Ukrainian Ministry of Defense said "combat and other military equipment, such as tanks, armed vehicles and 'Iskander' missiles remain near the border of Ukraine," following numerous Russian military exercises.

Russia regularly "redeploys and accumulates its military units to maintain tension in the region," the assessment said, allowing it to "create striking forces rapidly, and lays the ground for quick reinforcement."

Months of steady military increases along the Russia-Ukraine frontier have alarmed Ukrainian and Western officials. CNN reported last week that Russian forces had capabilities in place along the Ukrainian border to carry out a swift and immediate invasion, including erecting supply lines such as medical units and fuel that could sustain a drawn-out conflict.

However, Russian President Vladimir Putin has described the suggestion that Russia would invade Ukraine as "provocative."

"Russia is pursuing a peaceful foreign policy. But it has the right to ensure its safety," he said during a televised news conference on Wednesday, emphasizing the Kremlin's concern at the prospect of Ukraine one day joining NATO.

In a virtual meeting Tuesday that turned tense at times, US President Joe Biden warned Putin about invading Ukraine, and Washington is preparing a series of harsh economic sanctions in the event that Russian forces do attack.

Biden ended the conversation without any further clarity on whether Putin had made up his mind to launch an invasion, US officials said afterward.

Ukrainian Defense Minister Oleksiy Reznikov on Monday said the build-up of Russian troops was an attempt to spread fear and unrest in Ukraine, and to prevent Kiev from moving closer toward NATO and membership of the European Union.

"This gathering of their troops alongside of our border, it's a main goal of them to make destabilization process inside of our country, to stop us in our way.

But we go into the NATO ally, we're going to EU," Reznikov told CNN.

According to the latest Ukrainian assessment, Russia is also stepping up operations from the neighboring country of Belarus, which borders Ukraine to the north. The report said some units of the Russian army remain near the Russian border with Belarus "at the distance of around 260 km (160 miles) from the state border of Ukraine."

Russia has also intensified its intelligence gathering on Ukraine, the report added, with the number of reconnaissance aircraft flights along the Ukrainian border, above the Black Sea and the Sea of Azov, tripling since the same period last year.

Biden on Wednesday ruled out sending US troops to defend Ukraine in case of a military escalation, though the United States will send small arms and ammunition to Ukrainian defense forces this week as part of a $60 million security assistance package approved earlier this year, according to Pentagon spokesman John Kirby.

Ukrainian President Volodymyr Zelensky is expected to speak with Biden on Thursday.