Ukraine calls up reservists amid tensions with Russia

A Ukrainian border guard stands ready at the border from Russia to Ukrainian side of the Ukraine - Russia border in Milove town, eastern Ukraine, Sund

A Ukrainian border guard stands ready at the border from Russia to Ukrainian side of the Ukraine - Russia border in Milove town, eastern Ukraine, Sund

Ukrainian servicemen are seen aboard a naval ship in the Azov Sea port in Mariupol, Ukraine, Sunday, Dec. 2, 2018.  Ukraine's leader called on Germany

Ukrainian servicemen are seen aboard a naval ship in the Azov Sea port in Mariupol, Ukraine, Sunday, Dec. 2, 2018. Ukraine's leader called on Germany

Ukrainian servicemen are seen at a naval ship in the Azov Sea port of Mariupol, Ukraine, Sunday, Dec. 2, 2018. Ukraine's leader called on Germany and

Ukrainian servicemen are seen at a naval ship in the Azov Sea port of Mariupol, Ukraine, Sunday, Dec. 2, 2018. Ukraine's leader called on Germany and

A Ukrainian border guard checks documents of a woman crossing the border the Ukraine - Russia border in Milove town, eastern Ukraine, Sunday, Dec. 2,

A Ukrainian border guard checks documents of a woman crossing the border the Ukraine - Russia border in Milove town, eastern Ukraine, Sunday, Dec. 2,

KIEV, Ukraine (AP) — Ukraine's president has announced a call-up of reservists amid tensions with Russia.

Relations between the two neighbors have strained further following the Nov. 25 incident in which Russia fired upon and seized three Ukrainian naval vessels and their crews.

Ukrainian President Petro Poroshenko responded by introducing martial law for 30 days in much of Ukraine.

Poroshenko said Monday that reservists will be summoned for training as part of martial law. He also said that some military units will be redeployed to strengthen the nation's defenses.

Ukraine and Russia have blamed each other for the naval incident that further escalated the tug-of-war that began in 2014 when Russia annexed Ukraine's Crimean Peninsula and supported separatist rebels in eastern Ukraine.