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KIEV (Reuters) - Ukraine tested its Javelin missile systems on Tuesday for the first time since receiving the weapons from the United States to boost its defenses in its conflict with pro-Russian separatists.

Washington has been one of Kiev’s staunchest supporters since Moscow’s annexation of Crimea in 2014 and the subsequent outbreak of fighting in Ukraine’s eastern Donbass region that has killed more than 10,000 people.

Kiev and Washington believe the anti-tank systems will help Ukraine build its long-term defense capacity.

The United States says Javelins cannot effectively be used to take territory, but Russia says supplying them will further destabilize the situation by encouraging the use of force.

Ukrainian President Petro Poroshenko announced the first test of the missiles on Twitter: “Finally this day has come,” he said. “I am grateful to (the United States)... for the firm decision that Ukraine should be armed.”

The tests were carried out in northern Ukraine, away from the eastern front line. Intermittent clashes in eastern regions have continued despite a notional ceasefire and diplomatic peace efforts.

The Organization for Security and Cooperation in Europe (OSCE), which monitors the implementation of Ukraine’s much-violated ‘Minsk’ peace agreement, warned on Tuesday of an escalation in fighting along the front line.

“I call strongly upon the sides to immediately stop the fighting ...and do their utmost to protect the civilian population,” OSCE Special Representative Martin Sajdik said in a statement.