By Adrian Croft

BRUSSELS (Reuters) - NATO will triple its usual number of fighter jets patrolling over the Baltics next month to beef up its eastern European defenses due to tension with Russia over Ukraine, a NATO military official said on Tuesday.

Four NATO fighters are usually based in the Baltics but the United States, which currently has responsibility for patrolling the skies there, increased that to 10 F-15s to reassure those states following Russia's occupation of Ukraine's Crimea region.

The number of fighters assigned to the Baltic "air policing" mission is now set to increase further to 12, three times the normal complement, when the next four-month period starts in May, the NATO military official said.

The strengthening of air patrols over the Baltics is part of a plan of air, land and sea reinforcements for central and eastern European allies that NATO's top military commander, U.S. Air Force General Philip Breedlove, is drawing up by April 15.

Since the three ex-Soviet republics in the Baltics joined NATO in 2004, other NATO members have taken turns sending fighter aircraft to protect their airspace since Estonia, Latvia and Lithuania lack fighters of their own.

The jets are on call to respond to any violations of Baltic airspace. Lithuania said last week the number of Russian jets flying close enough to Baltic airspace this year to prompt NATO jets being scrambled has increased to around one a week.

Poland takes over responsibility for providing Baltic air policing from May 1, offering four MiG 29s.

SECOND BASE

Britain will contribute four Typhoons and Denmark has offered four F-16s as well as another two F-16s that will be on standby in Denmark, the NATO military official said.

Until now, the Baltic air policing mission has operated from an air base in Lithuania but Estonia has now offered Amari as a second base that the four Danish planes are expected to use.

France has also offered four fighters, either Rafales or Mirages, for the May to August period.

Story continues

"We are still finalizing the details of French participation. What is likely is that they will fly out of Poland," the official said.

The French aircraft would probably not be directly involved in Baltic air policing but may carry out training flights over Poland, he said. Poland has been asking for a greater NATO presence on its territory.

Germany has offered six Eurofighters for the Baltic mission but NATO will probably ask Berlin to delay its participation until the following four-month period from September to December when Portugal takes over responsibility for the mission.

In addition to the Baltic reinforcements, France has offered an AWACS reconnaissance plane to join NATO AWACS aircraft that have been flying over Poland and Romania since last month to monitor the situation in Ukraine.

The French plane made its first flight on April 1, the official said.

The United States and the Netherlands have provided refueling planes for the AWACS flights and Turkey has also offered a tanker aircraft that is expected to join the operation soon.

(Additional reporting by Nerijus Adomaitis in Oslo, Andrius Sytas in Vilnius, Kylie MacLellan in London and John Irish in Paris; Editing by Tom Heneghan)