Moscow: Russian President Vladimir Putin on Friday discussed the aggravation of the crisis in Ukraine in a telephonic conversation with his German and French counterparts Angela Merkel and Francois Hollande. In the conversation, the leaders expressed concern over the recent increase in ceasefire violations in the eastern Ukrainian region of Donbass, Xinhua news agency quoted the Kremlin as saying in a statement. The leaders deemed it a priority to prevent further degradation of the situation in the conflict zone, through consistent implementation of agreements on withdrawal of heavy weapons from the front line and separation of the opposing forces, the statement said. It stressed the Special Monitoring Mission of the Organisation for Security and Cooperation in Europe (OSCE) should effectively ensure truce in the region. Putin drew attention of the leaders of Germany and France to "provocative actions of Ukrainian troops", urging them to put more pressure on Ukrainian President Petro Poroshenko to ensure the implementation of the Minsk agreements by the Kiev government. The leaders stressed the need to intensify negotiations in various formats for the full and comprehensive implementation of the Minsk accords aimed at reaching a settlement of the crisis, agreed in the capital of Belarus in February 2015. US Secretary of State John Kerry on Thursday visited Ukraine on the eve of the NATO summit, which started in Warsaw on Friday. Kerry said full implementation of the Minsk agreements to resolve the military conflict in eastern Ukraine may start before the end of this year.