BELGRADE (Reuters) - Paratroopers from Serbia and the United States jumped side-by-side just north of Belgrade on Friday as part of a training tailored to bolster ties between the Balkan country and NATO. The exercise dubbed Winter 2017, may raise eyebrows in Moscow which wants to keep Serbia, a Slavic and Orthodox Christian ally under its umbrella, and is worried over possible NATO expansion in the Balkans. It is also a long way from 1999 when NATO bombed what was then Yugoslavia because of its policies toward Kosovo. Although the European Union is Serbia s single largest trade partner and investor, Russia controls its oil and gas supplies. Moscow has also sought to bolster military ties with Belgrade with the donation of six MiG-29 fighter jets. Serbia is one of the few Balkan countries not in the 28-member NATO which is hugely unpopular there due to its 1999 bombing campaign. Alliance troops are still deployed in Kosovo, Serbia s former southern province, whose 2008 independence has not been accepted by Belgrade, Russia, China and some EU members. But joint exercises take place anyway. After more than 100 paratroopers from Serbian elite brigades and the U.S. Army 173rd Airborne Brigade landed on a grassy runway of a small airport near Belgrade, Serbian President Aleksandar Vucic dubbed the drill part of confidence-building activities.   The other (goal) is the creation of cooperation, partnerships and friendships (in areas that) we were not always able to boast about in the past,  Vucic told reporters. In 2006 Serbia, which maintains military neutrality, joined NATO s Partnership for Peace program and in 2015 it signed the Individual Partnership Action Plan - the highest level of cooperation for countries not aspiring to join.  Although it strives for a balance between Moscow and the West, rejecting calls from U.S. officials to pick its side, Serbia in 2017 had 13 military drills with NATO or its members, seven with the U.S. and only two with Russia. 