WASHINGTON, July 14 (Xinhua) -- The United States on Monday voiced its concern over Russia's violation of Georgian airspace by sending military aircraft over the rebellious Georgian region of South Ossetia.

    "We are deeply troubled by Russia's statement that its military aircraft deliberately violated Georgia's internationally recognized borders," State Department spokesman Sean McCormack said in a statement.

    Russia said Friday that four of its planes circled over South Ossetia late Wednesday for about 40 minutes to head off a possible "invasion" by Georgian troops. Georgia threatened to shoot down Russian planes if the incursion is repeated.

    Georgia's breakaway regions have long been a source of tension between Russia and Georgia. Bilateral relations were further strained recently after Moscow deployed additional peacekeepers in Abkhazia.

    "The United States is concerned by the recent escalation in violence in the Georgian regions of South Ossetia and Abkhazia and calls upon all sides to return to direct negotiations and resolve their differences peacefully," McCormack said.

    South Ossetia and Abkhazia broke away from central rule in the 1990s following the collapse of the former Soviet Union. But their self-proclaimed independence is not internationally recognized.