LVIV, Ukraine, July 27 (Reuters) - Helicopters provided air support, armoured personnel carriers rolled through fields, and soldiers fired at enemy targets on Tuesday as part of a large military exercise hosted by Ukraine and also involving the United States, Poland and Lithuania. The drills, called Three Swords-2021, will involve more than 1,200 servicemen and more than 200 combat vehicles and will last from July 17 to 30 at Yavoriv training ground in Lviv region. "You have seen several tactical exercises," said Yevhen Moysyuk, commander of the Ukrainian air assault forces. "They show a high level of preparedness and the readiness of our partners to jointly repel aggression," he told a briefing. The drills come amid increased tensions between NATO and Moscow, which said in June it had fired warning shots and dropped bombs in the path of a British warship to chase it out of Black Sea waters off the coast of Crimea. Relations between Kyiv and Moscow plummeted after Russia seized Crimea and over Russia"s support for separatist forces in the conflict in Ukraine"s eastern Donbass region. Ukraine and the United States in June held a separate military exercise, Sea Breeze, involving more than 30 countries in the Black Sea and southern Ukraine, despite Russian calls for the drills to be cancelled.