"NATO remains vigilant. We are monitoring this situation very closely," Stoltenbeg said after a meeting with Ukraine's foreign minister.

"Any further provocation or aggressive actions by Russia would be of serious concern. We call on Russia to be transparent about its military activities," Stoltenberg said.

He added that it was "important to prevent escalation and reduce tensions".

The United States has been voicing increasing concern to allies over the recent Russian troop movements at the frontier with Ukraine.

Secretary of State Antony Blinken said last week it would be a "serious mistake for Russia to engage in a repeat of what it did in 2014", when it seized Crimea from Kiev.

And on Monday, Germany's foreign ministry urged Moscow to exercise "restraint" at the Ukrainian border.

Russia sparked major alarm at NATO back in spring that it could be looking to intervene in Ukraine with a huge build-up of troops at the border.

Ukrainian Foreign Minister Dmytro Kuleba said the latest movements were a continuation of the earlier pressure his country had faced.

"In Ukraine's view, what we currently see along the border is not a purely military buildup, because Russia already brought a military armada to our borders in spring and has never really withdrawn it since," he said.

"What we see now is a deteriorating situation where Russia is demonstrating that it can quickly activate the already amassed troops and equipment and that every option, including the military one, is on the table for Russian leadership."

Kuleba said that Kiev was looking to closely coordinate with its Western allies to "take any measures which can help to deter Russia and to prevent the worst case scenarios".

Ukraine remains locked in a festering conflict with Russia-backed separatists in the east of the country that started seven years ago.