After monitoring "large and unusual gatherings of Russian troops" on the Ukrainian border in recent weeks, NATO Chief Jens Stoltenberg warned Moscow against any acts of aggression.

After a meeting with the Ukrainian Foreign Minister, Stoltenberg said, "Any provocation or other acts of aggression by Russia will be a serious concern. We call on Russia to be transparent about its military activities. "

The United States, the European Union and France have expressed concern in recent days about the deployment of Russian forces on the Ukrainian border. Germany called on Moscow for "restraint."

Calling on Russia to "exercise restraint" and "return to the negotiating table," a German Foreign Ministry spokesman in Berlin said: "We must prevent a military escalation."

In 2014, Russia annexed Ukraine's Crimea and subsequently, Ukraine engaged in armed conflict with pro-Russian separatists in the east of the country, as Kiev and the West accuse Russia of supporting separatists militarily and financially, which Moscow denies.

"What we are seeing along the border is not a purely military reinforcement because Russia deployed a military fleet to our border in the spring and has not withdrawn it since," said Dimitro Koleba.

From NATO headquarters in Brussels, he added: "What we see now is a deteriorating situation in which Russia proves that it can quickly activate the troops and equipment it has mobilized and that all options, including military, are on the table of Russian leaders."

For his part, Stoltenberg said that NATO was "closely monitoring the situation," reiterating that the Allies considered the annexation of Crimea "illegal."

All NATO member States "condemned with one voice the behaviour of Russia."