KIEV, July 20 - Ukrainian and U.S. troops launched fresh drills Monday near the war-torn country's Polish border in a bid to show unity and resolve in the face of an increasingly defiant Kremlin.

The annual Rapid Trident exercises involve 1,800 soldiers from 18 countries and will last for just under two weeks, AFP reported.

Their immediate aim is to build morale and cohesion within the ex-Soviet state's outdated and woefully underfunded armed forces -- caught in a 15-month east Ukrainian quagmire battling pro-Russia militias that has claimed more than 6,500 lives.

But they also deliver a transparent message to the Kremlin about Washington and its allies' determination to thwart any expansionist ambitions Russian President Vladimir Putin may have.

"These joint maneuvers... display a broad support for Ukraine in its struggle for freedom and sovereignty," Ukrainian forces commander Oleksandr Syvak told the festive flag-raising ceremony.

His US counterpart Alfred Renzi said the participating countries -- most of them NATO members but also including such former Soviet nations as Moldova and Azerbaijan -- "will prove an ability to cooperate as one unified force for stability."

Russia quickly condemned the war games as a threat to an already shaky five-month truce agreement that aims to resolve one of Europe's bloodiest conflicts in decades by the end of the year.

NATO "should understand that such actions... may threaten to disrupt the visible progress (made) in the peace process concerning the deep internal crisis in Ukraine," the Russian foreign ministry said.

Putin has always denied charges of orchestrating Ukraine's separatist revolt to unsettle the pro-Western leadership that rose to power in the wake of last year's ouster of a Moscow-backed president.

But the veteran Russian leader -- immensely popular at home for his patriotic fervour while increasingly isolated abroad -- has done little to mask his views of eastern Europe being part of Moscow's traditional sphere of influence.

The launch of Rapid Trident was quickly followed by the Russian navy's announcement that one of its warships stationed off Ukraine's Kremlin-annexed Crimean peninsula would conduct live rocket fire drills Sunday.

"After a long interruption and in order to demonstrate the navy's combat capabilities, the Ladny frigate will attack a dummy target from an anti-submarine system," Black Sea Fleet spokesman Vyacheslav Trukhachyov told Moscow's news agencies. (afp/ez)