* Large U.S.-led exercises are under way in Baltic region
* Latvia says NATO fighters scramble to track Russian planes
* NATO says its measures are "purely defensive" (Adds NATO fighters scrambled to track Russian planes near Latvia)
MOSCOW, June 12 (Reuters) - Russia has begun military exercises in its Baltic enclave of Kaliningrad in what the Defence Ministry said was a response to drills by NATO allies in parts of eastern Europe that were launched after Moscow's intervention in Ukraine.
In a further sign of friction between Russia and NATO, Latvia said on Thursday that NATO fighters that carry out air patrols over the Baltics scrambled a record four times on Thursday after a total of 16 Russian military planes were spotted close to Latvia's air space.
Russia's annexation of Ukraine's Crimea region and a pro-Russian separatist revolt in the country's east after its Moscow-backed president was ousted by protesters have led to the worst East-West crisis since the end of the Cold War.
The Defence Ministry did not reveal the scale of the Russian exercises but said the equipment and number of troops involved "corresponds" to the size of the NATO manoeuvres.
"The training of the army's group in the Kaliningrad operational (theatre) is being held simultaneously with the international (NATO) exercises of Saber Strike-2014 and BALTOPS-2014 launched in Europe," its statement said.
Kaliningrad is a sliver of territory that is unconnected to the rest of Russia and sandwiched between NATO member states Lithuania and Poland.
NATO countries responded to the annexation of Crimea by sending fighter planes and ships to eastern Europe to reassure allies alarmed by Russia's action.
The U.S.-led alliance and its member states have also stepped up military exercises in eastern Europe, including the three former Soviet republics in the Baltics - Estonia, Latvia and Lithuania - that now belong to the Western alliance.
Two large U.S.-led military exercises are under way in the Baltic region.
A U.S.-led exercise, named "Saber Strike", involving around 4,700 soldiers from 10 countries, is taking place in Lithuania, Latvia and Estonia. Forces from 13 countries are participating in a U.S.-led naval exercise in the Baltic Sea, dubbed BALTOPS.
Russia's Defence Ministry said that 24 ships from Moscow's Baltic Sea Fleet were patrolling Russian territorial waters while its regional air force had been beefed up with extra Su-27 fighter jets.
In a development that may be related to the Russian exercises, the Latvian army said NATO fighters that guard the airspace of the three Baltic states scrambled four times on Thursday to identify Russian military planes flying close to Latvia's air space.
A total of 16 Russian military aircraft were spotted in the area on Thursday. The number of aircraft spotted and the number of times the NATO fighters had scrambled were both the biggest yet recorded in a single day, the Latvian army said.
That appears to mark a resurgence of Russian probing of NATO's air defences in the Baltics after a lull in recent weeks since NATO reinforced its security measures.
Lithuania said in April that the number of Russian jets flying close enough to Baltic airspace to prompt NATO jets being scrambled had increased to around once a week in January and February. But the U.S. ambassador to NATO, Douglas Lute, said later in April that Russian incursions or near incursions in the Baltics had fallen to zero since NATO beefed up air cover.
Russian President Vladimir Putin's declaration in March that he had the right to invade Ukraine to protect Russian speakers there, caused alarm in Estonia and Latvia, which have large ethnic Russian minorities of their own.
Asked this week about reports of Russian exercises in Kaliningrad, NATO Secretary-General Anders Fogh Rasmussen said he understood Russia considered its exercises to be a response to the measures NATO had taken to step up its security.
But NATO's actions were "purely defensive measures that do not justify any offensive action", he told a Brussels conference. (Reporting by Vladimir Soldatkin in Moscow, Adrian Croft in Brussels; and Aija Krutaine in Riga; Editing by Alison Williams)