WARSAW (Reuters) - Lithuania said a growing rift between some eastern and western European Union states over issues such as migration posed a threat to the bloc at a time of increased Russian military assertiveness. Frictions between ex-communist states in Europe s East and the wealthier West have increased since the 2015 migration crisis and Britain s decision to leave the bloc, as leaders try to quell popular disenchantment with the EU. Nationalist politicians in Poland and Hungary have called for sweeping reform to bring more power back to member states at the expense of Brussels bureaucracy and refused to take part in efforts to relocate migrants from the Middle East.   I believe it s a worry,  Lithuanian Foreign Minister Linas Linkevicius told Reuters on the sidelines of a security conference in Warsaw, when asked about Polish and Hungarian assertiveness within the EU.  We would like to see more cohesion,  he said.  I know who is gaining. Those who are not happy with our cohesion,  Linkevicius said, adding that he was referring to Russia.  We are taking it very seriously not to help those who would like to divide East and West.  Lithuania, alongside Poland, has been particularly worried about Russia since Moscow annexed the Crimean peninsula from Ukraine in 2014.  NATO has sought to reassure allies in the region by sending troops to the Baltics, Poland and the Black Sea, setting up a network of NATO outposts, holding more exercises and preparing a rapid response force. Some western officials have expressed concern that parts of the Baltic states, which have large ethnic Russian minorities, could be seized by Moscow, much as Russia took control of Crimea. Linkevicius said good relations with EU powerhouses Germany and France within the EU were crucial because of their ability to help militarily.  Poland, in particular, has seen ties with Paris and Berlin deteriorate since 2015, when the conservative Law and Justice (PiS) party took power, over issues such as military procurement, wartime reparations and the EU s single market rules.  