STOCKHOLM (Reuters) - Sweden said two Russian warplanes entered its airspace this week, calling the intrusion a "serious violation" and sending a protest to Moscow's ambassador in the Nordic country.

Countries in the region have become increasingly wary of Russia's military ambitions since Moscow annexed Ukraine's Crimea region in March.

Western powers have also accused Russia of sending troops to back rebels in eastern Ukraine - a charge it denies.

Sweden's foreign ministry said on Friday the two Russian warplanes entered Swedish airspace south of the island of Oland in the Baltic Sea on Wednesday.

The news is likely to stoke a debate on strengthening Sweden's defenses. The outgoing center-right government, which was defeated in parliamentary elections this month, said this spring it planned to increase defense spending, citing concerns over Russia's actions.

The Social Democrats, who are likely to lead a new government, have also said they want to bolster Sweden's defense.

(Reporting by Sven Nordenstam; Editing by Andrew Heavens)