“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.