BELGRADE, Serbia (AP) — Russian military cooperation with Serbia has reached a “fundamentally” new level, Russia’s defense minister said Monday as he visited his Balkan ally amid rising tensions in the volatile region.

Sergei Shoigu said in Belgrade on Monday: “For our part, we will make every effort to maintain the pace of our relations in the military sphere.”

Serbia remains a key ally of Russia even though it wants to join the European Union. Belgrade has pledged to stay out of NATO and refused to join Western sanctions against Russia over the conflict in eastern Ukraine.

Russia has been helping Serbia beef up its military with fighter jets, attack helicopters and battle tanks, raising concerns in the war-scarred Balkan region of possible new clashes after a bloody civil war in the 1990s.

Serbia's flirting with Russia, as well as with China, is seen with unease in the West.

During his visit, Shoigu was expected to announce the delivery to Serbia of a sophisticated anti-aircraft missile system even though the U.S. has warned of possible sanctions against Serbia in the event of such delivery.

“A strong army is the guarantor of the state’s sovereignty,” said Shoigu according to the Russian TASS news agency. “Serbian citizens can be rightfully proud of their armed forces."

Serbia's pro-Russian Defense Minister Aleksandar Vulin greeted Shoigu as a “true friend of Serbia" and said the relations with Russia are “on the highest historical level."

During the bloody breakup of Yugoslavia in the 1990s, Serbia was at war with neighbors Croatia, Bosnia and Kosovo. Currently, its nationalist leadership is believed to be behind the latest tensions in neighboring Montenegro and Bosnia.

In Montenegro, the newest NATO member, minority Serbs have been demonstrating for months against a law that they say threatens the existence of their Serbian Orthodox Church. In Bosnia, a Serb pro-Russian leader is threatening to split from the joint state with Muslims and Croats and join about a third of the country with Serbia.