LONDON (Reuters) - Britain said on Monday it was concerned by a sharp increase in Russian military flights close to its borders which it said were designed to test its air defenses.

"The extremely aggressive probing of our air space by Russian aircraft is an issue of concern to us," British Foreign Secretary Philip Hammond told a committee of lawmakers.

"It is entirely appropriate for NATO countries in particular to work together to respond to what is a change of Russia's dealings with NATO and indeed the non-NATO European countries."

Interceptions of Russian military aircraft by the military alliance's planes have increased since Moscow annexed Ukraine's Crimea region in March, in particular over the Baltic region.

Last month, Britain's Royal Air Force intercepted Russian military aircraft flying close to British air space at least twice, an occurrence which routinely triggers the scrambling of fighter jets to escort them away.

Earlier this year, Hammond, then defense secretary, said Britain had observed a resurgence in Russian air activity in recent years because of a Kremlin military overhaul.

(Reporting by Kylie MacLellan; Editing by Andrew Osborn)