Reuters reports U.S. President Barack Obama urged NATO on Wednesday to help strengthen Ukraine's military and said the alliance must leave the door open to new members to counter what he called Russian aggression. Obama was speaking in Estonia, one of three ex-Soviet Baltic states bordering Russia that fear separatist rebellion in eastern Ukraine could herald problems for them. Obama also called on European states to increase their defense spending. Military spending among European NATO members was 1.6 percent of GDP last year, below the 2 percent goal and only three countries -- Britain, Estonia and Greece -- reached the minimum needed while several east European countries spent less than 1 percent. Speaking about concerns that Russia's expansion could hit the Baltics, where ethnic Russians make up a sizable minority, Obama said NATO would not hesitate to protect one of its members, even on the periphery of the alliance.