BELGRADE, Serbia (AP) — Russia increased pressure on Serbia on Tuesday to grant diplomatic status to the Russian staff of a controversial facility that U.S. officials believe could become Kremlin's spy base in the Balkans.

Moscow has repeatedly denied reports that the so-called "Russian-Serbian Humanitarian Center" in the southern city of Nis was a base for Russia to eavesdrop on the West in the Balkans — a tense region where Russia is vying for influence. Moscow says the center is only for humanitarian missions.

Serbian President Aleksandar Vucic's office said that during a meeting on Tuesday, Russian Ambassador to Serbia Alexander Chepurin urged him to resolve the status of the center "as soon as possible" because of "the volume of its work."

Serbian Prime Minister Ana Brnabic has said that "neither Moscow nor Washington" will influence her government's decision on the center's status.

"The center in Nis is very important for our country because that's where the aid starts in emergency situations," she said.

The U.S. has expressed concern about the center, which Russia and Serbia jointly opened in 2011. Its location is close to Serbia's border with NATO member Bulgaria and Kosovo, which houses a large American military base and the Western alliance's peacekeeping troops.

The Russian Ministry for Emergency Situations is a partner in the center. The semi-military outfit's activities do include disaster relief, but the agency also carries out jobs for Russia's security services.

Serbia has been boosting military, economic and other ties with its traditional Slavic ally Russia, while formally seeking European Union membership.

The U.S. deputy assistant secretary of state for European and Eurasian affairs, Hoyt Brian Yee, told a recent Senate hearing on the Balkans that the existence of the base does not worry the Americans "for what it is now, but what it might become if it receives what Russia has been asking from Serbia, what is some kind of special status, a protected diplomatic status or immunity."