Russia could attack a Nato country within the next five years, the Western military alliance's chief has said in a stark new warning.

The Kremlin has described Ukraine’s push to join NATO as one of the “root causes” of the conflict. Russia sees an expanding NATO as a major security threat. But NATO’s expansion into the Baltics and other post-Soviet states is driven by a fear that Moscow would one day invade and try to fold them back under its control—as seen in Ukraine.

European leaders are expected to cement support for Ukraine Monday as it faces Washington’s pressure to swiftly accept a U.S.-brokered peace deal.