BERLIN—German Chancellor Angela Merkel on Monday called for a full withdrawal of Russian armed forces from eastern Ukraine and expressed concern about repeated breaches of the cease-fire there by pro-Russia separatists.

The effective implementation of the recent cease-fire in eastern Ukraine and the launch of a political process to solve the conflict couldn't take place as long as the Russian military remained deployed in the region and the border between the two countries wasn't secured, Ms. Merkel told Ukrainian President Petro Poroshenko in a telephone conversation on Monday, according to the chancellor's spokesman.

The chancellor told Mr. Poroshenko she shared his concerns about " recurrent breaches of the cease-fire by separatists," Ms. Merkel's spokesman Steffen Seibert said in a statement. Ukrainian government and rebel representatives agreed on a cease-fire early this month after Russia ramped up its support for the separatists, stopping a weekslong advance by the Ukrainian army in the region.

German government officials have grown concerned about Russia's persistent military presence in eastern Ukraine and have described the cease-fire as very fragile.

Write to Bertrand Benoit at bertrand.benoit@wsj.com