President Vladimir Putin on Thursday said Russia had been forced to respond to NATO enlargement and that its annexation of Crimea, home to its Black Sea Fleet, was partly influenced by the Western military alliance's expansion into eastern Europe, Reuters reported.
"When the infrastructure of a military bloc is moving toward our borders, it causes us some concerns and questions. We need to take some steps in response," Putin said in a televised call-in with the nation.
"Our decision on Crimea was partly due to ... considerations that if we do nothing, then at some point, guided by the same principles, NATO will drag Ukraine in and they will say: 'It doesn't have anything to do with you.'"
The vast majority of residents of Crimea - 96 percent - voted to secede from Ukraine and join Russia, in a referendum held March 16.
With the exception of Russia most countries refused to recognize the referendum and its results.
On March 18, Russia and Crimea signed an agreement on Crimea as well as Sevastopol city joining the Russian Federation.
A change of power took place in Ukraine on February 22.
The Verkhovna Rada (parliament) of Ukraine ousted President Viktor Yanukovych from the power, changed the constitution and scheduled presidential elections for May 25.
Yanukovych said he was forced to leave Ukraine under the threat of violence, and he remains the legally elected head of state.
A number of provinces in eastern and southern Ukraine, as well as the Crimea did not recognize the legitimacy of the Rada and decided on possibility of holding a referendum on the future fate of the regions.