Confrontation in Ukraine as diplomacy stalls

* Russian forces tighten grip on Crimea

* Ukrainian PM to address U.N. Security Council

* Lavrov says Kerry declined offer of talks

By Andrew Osborn and Natalia Zinets

SEVASTOPOL/KIEV, March 10 (Reuters) - A pro-Russian force opened fire in seizing a Ukrainian military base in Crimea on Monday and NATO announced reconnaissance flights along its eastern frontiers as confrontation around the Black Sea peninsula showed no sign of easing.

Ukrainian activists trying to cross into Crimea to show solidarity with opponents of last week's Russian military takeover there said they were halted by men in uniforms of the now outlawed riot police. One of these fired at close range, hitting a man in the chest, apparently with rubber bullets.

With diplomacy at a standstill, Russia said the United States had spurned an invitation to hold new talks on resolving the crisis, the worst East-West standoff since the Cold War - although Washington said later a meeting of foreign ministers was possible this week, if Moscow shows it is ready to "engage".

The U.S.-led NATO defence alliance said AWACS early warning aircraft, once designed to counter feared Soviet nuclear missile strikes, would start reconnaissance flights on Tuesday over Poland and Romania to monitor the situation in Ukraine, flying from bases in Germany and Britain.

The United States on Tuesday will also begin previously planned military training exercises in the region, the first since the Russian intervention in Crimea. A U.S. Navy destroyer will participate in maneuvers with Romanian and Bulgarian warships in the Black Sea, across from Crimea. In Poland, U.S. fighter jets will take part in joint exercises.

British Prime Minister David Cameron told Germany's Bild newspaper, however, that Western powers were not considering military action and wanted a diplomatic solution. European Union governments are considering sanctions against Russia.

Ukrainian Prime Minister Arseny Yatseniuk, who said he would address the U.N. Security Council on Thursday, blamed the crisis on Russia and accused Moscow of undermining the global security system by taking control of Crimea.

Ukraine's new justice authorities issued warrants for the arrest of Crimea's pro-Russia leaders on Monday, six days before a referendum they have called to join the region to Russia.

Russian forces have in little more than a week taken over military installations across Crimea, home to the Russian Black Sea Fleet and Russian territory until Soviet leader Nikita Khrushchev gave it to Ukraine in 1954.

Pro-Russian separatists have taken control of the regional parliament, declared Crimea part of the Russian Federation and announced a referendum for Sunday to confirm that.