The head of Ukraine's border services is warning of a Russian military build-up, a day after Kiev pulled its forces from Crimea.
As Ukrainian troops pulling out from Crimea and the last military bases in the region fall under Russian control, fears of a further Russian incursion into the Ukrainian mainland are growing in Kiev.
Volodymyr Lytvyn is the head of Ukraine's Border Guard service.
"The build-up of the Russian Federation military force continues. At the beginning they were building up for the military exercises, now when the exercises are over they are still at their locations which we clearly see and know. These troops are maneuvering every day near our state border. There are constant provocations near the state border, especially at night time."
Moscow is dismissing any suggestion that its mobilizing its troops along its border with Ukraine.
The Russian military did stage an exercise involving 85-hundred soldiers near its border with Ukraine after the toppling of pro-Moscow Ukrainian president Viktor Yanukovich.
Amid the continuing tension along the Ukrainian-Russian border, observer teams from the Organization for Security and Co-operation in Europe have begun deploying across Ukraine.
Meanwhile, authorities in Kiev have announced they have no plans to become a nuclear weapons state again.
New calls for Ukraine to recover its nuclear-state status surfaced last week after several parties in the Ukrainian parliament introduced a bill, calling on the country to exit the Non-Proliferation Treaty in response to Crimea's absorption into Russia.
In 1994, Ukrainian authorities signed the Budapest Memorandum, under which Ukraine gave up the world's third-largest nuclear weapons stockpile in exchange for a guarantee of its sovereignty and unity.
Russia, the United States and Britain, as guarantors of the treaty, are obliged not to use force against Kiev and mediate if a threat to Ukraine's territorial integrity arises.
Russia has rejected claims that it has violated the Budapest Memorandum by taking control of Crimea.
