BELGRADE, Serbia — American and Serbian paratroopers held joint military exercises Friday in Serbia, watched with unease by Russia, which is trying to increase its influence in the Balkans and keep the country within its fold.

Serbian President Aleksandar Vucic attended the last day of the four-day drill that included joint jumps by Serbian and U.S. parachutists from two U.S. Air Force C-130J Hercules transport planes that flew close to the Serbian capital, Belgrade.

“The joint exercise contributes to the (military) skills, but also enhances partnership and friendship that was not always seen in the past,” Vucic said. “I’m grateful to our American partners who have showed that in a short time we could organize these activities.”

In 1999, a 78-day U.S.-led NATO bombardment ended a Serbian crackdown against ethnic Albanian separatists in its former province of Kosovo, making the Western military alliance very unpopular among the Serbs.

John Gronski, the U.S. Army Europe deputy commanding general, said after the drills that such exercises with the Serbian forces “build the readiness of both of our militaries and when you have ready military, a region can be more stable and secure. “

American and NATO-related military drills in the Balkans regularly trigger anger by the Kremlin, which opposes its expansion in the former communist Eastern Europe. Serbia is considered the last remaining Russian ally in the region.

Serbia, which tries to politically balance between Russia and the West while seeking European Union membership, claims military neutrality. But Moscow has been arming the country with fighter jets and other equipment, worrying neighboring states in the region that saw a bloody civil war in the 1990s.

NATO and Serbia have been improving cooperation since the country joined its outreach Partnership for Peace program in 2006.

“I believe that we will improve (our relations) in the future,” Vucic said, adding that “Serbia will, understandably, jealously preserve its military neutrality.”

Gronski, the U.S. general, said whether Serbia eventually joins NATO depends on politicians.