WARSAW (Reuters) - Ukraine, Poland and Lithuania launched a joint military force on Friday that Polish President Bronislaw Komorowski said could start its first exercises in the tense region in the next year.

The three countries and other states in the area have been on high alert since Russia annexed Ukraine's Crimea region in March - and Western powers accused Moscow of sending troops to back rebels in eastern Ukraine.

Polish defense officials said the new joint unit could take part in peacekeeping operations, or form the basis of a NATO battle group if one was needed in the future.

"I hope that it will be possible to hold first exercises in one year," Komorowski said after the three countries' defense ministers signed an agreement setting up the unit.

"This is part of a wider plan ... to support Ukraine, among others, in the area of modernization," he added.

NATO has already approved wide-ranging plans to boost its defenses in eastern Europe, aiming to reassure allies nervous about Russia's military ambitions. Moscow has denied it is arming pro-Russian rebels in eastern Ukraine.

The unit will be headquartered in the eastern Polish city of Lublin, not far from the border with Ukraine.

The size of the unit was still being worked out, Polish defense ministry spokeswoman Anna Klis said. It would be fully operational within 24 months, she added.

(Reporting by Wiktor Szary; Writing by Marcin Goettig; Editing by Andrew Heavens)