SARAJEVO (Reuters) - The European Union peacekeeping force (EUFOR) in Bosnia warned its political leaders on Tuesday it was prepared to intervene at short notice should violence resume two decades after the end of its ethnic war that killed 100,000 people.

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Concerns are rising about increasing instability in the historically volatile Balkans including secessionist pressures in Bosnia, a parliamentary boycott in Montenegro and renewed tensions between Serbia and its former province of Kosovo.

“A lot has been achieved but a lot can be lost again,” Major General Anton Waldner said at a ceremony in the Bosnian capital Sarajevo marking his takeover of the command of EUFOR, which has 800 troops deployed in Bosnia.

“There is still an executive mandate which allows significant reinforcement at short notice of (EUFOR). I will not hesitate to call (on) these reserve forces if needed,” Waldner, an Austrian, said. “You, political leaders, have the most powerful key in your hands.”

EUFOR’s new operations chief, General James Everard, said there were “external influences at play in the wider Western Balkans region, and in Bosnia, (that) have the potential to challenge progress.

Western leaders have accused Russia of seeking to exploit diminishing EU leverage in the Balkans by manipulating political events in the region. Russia, which denies such allegations, is a historical ally of the Serbs.

“In the face of such challenges it is essential that we persevere together,” added Everard, a Briton.

Last year, Montenegro’s authorities accused a group of Serb and Russian nationalists of planning a coup during elections to get an opposition alliance into power.

Russia strongly opposes the former Yugoslav republic becoming a member of NATO, but rejected the accusations.

EUFOR first deployed in Bosnia in 2004, replacing the 60,000-strong NATO Stabilization Force (SFOR).

For its part, NATO had said that in case of any violent flare-up in Bosnia, it could quickly deploy military assets, mainly from Italy and Germany.Fears of fresh conflict in Bosnia have risen amid calls from Serbs for the secession of their post-war autonomous entity from Bosnia and their overwhelming vote to keep a national holiday that Bosnia’s central top court has ruled unconstitutional.

Post-war Bosnia’s two highly autonomous regions, the Serb Republic and the Bosniak-Croat Federation, largely eclipse a weak central cabinet in Sarajevo. This has meant that economic reforms and development often become hostage to ethnic politicking and conflicting visions of the nation’s future.