(ANSA-AFP) - BRUSSELS, 06 APR - NATO Secretary General Jens Stoltenberg told Ukraine's leader on Tuesday that the alliance has "serious concern" over Russian military activities around its southern neighbour and backs Kiev's sovereignty. Last week, Kiev accused Moscow of massing thousands of troops on its northern and eastern borders, as well as on the Crimean peninsula, which was annexed by Moscow in 2014. Stoltenberg said he had called President Volodymyr Zelensky "to express serious concern about Russia's military activities in and around Ukraine and ongoing ceasefire violations". "NATO firmly supports Ukraine's sovereignty an territorial integrity. We remain committed to our close partnership." The phone call from the NATO chief follows messages of solidarity for Ukraine from leading alliance powers the United States and Britain, and from the European Union. Last week the Pentagon said US forces in Europe had raised their alert status following the "recent escalations of Russian aggression in eastern Ukraine". The Kremlin has not denied the recent troop movements, but insisted that Moscow was "not threatening anyone". It has warned it will take take necessary "measures" in the event of any Western military deployment in Ukraine. (ANSA-AFP).