(Bloomberg) -- The head of the NATO military alliance urged Russia to prevent any escalation in the wake of its build-up of forces near the border with Ukraine.

“What we see is a significant, large Russian military build-up, we see an unusual concentration of troops and we know Russia has been willing to use these types of military capabilities before to conduct aggressive actions against Ukraine,” Jens Stoltenberg, secretary general of the 30-nation North Atlantic Treaty Organization, told reporters on Monday.

“We call on Russia to be transparent on their military activities, to reduce tensions and to prevent any escalation,” Stoltenberg said after talks with Ukrainian Foreign Minister Dmytro Kuleba in Brussels. “This is a clear message from all NATO allies.” 

The build-up of tanks and troops near the border with Ukraine has western capitals worried that Russian President Vladimir Putin could be planning a repeat of the 2014 invasion. In Moscow, senior officials and others close to the leadership say the Kremlin aims to make clear that any further western steps to provide weapons or expand military facilities in Ukraine would cross Russia’s red line. 

Stoltenberg declined to elaborate on the size or composition of the Russian deployment. “The fact that we see this military build-up also reduces any warning time between a decision in Russia before they’re able to conduct a military aggressive action against Ukraine,” Stoltenberg said.

“What we currently see along the border is not a purely military build-up because Russia has already brought a military armada to our borders in spring and has never really withdrawn since,” Kuleba said.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 a military one is on the table.”

©2021 Bloomberg L.P.