(QHA) -

US President Barack Obama and European Council President Donald Tusk Monday called for unity on Ukraine and discussed Europe's security challenges, including terrorism.

Obama saluted German Chancellor Angela Merkel and French President Francois Hollande for their work in establishing the Minsk cease-fire process.

"But we also know from experiences over the last year that unless we have strong monitoring and strong implementation, that these agreements will be meaningless,"- Obama said at the Oval Office meeting.

Today, we can see with full clarity that they are trying to divide us inside of Europe as well as Europe and America. But when we are united we will be able to put a stop to the aggressive policy of Russia against its neighbors"- Tusk said.

The EU and U.S. have moved hand in hand on sanctions against Moscow for its intervention in Ukraine, but Tusk said earlier in an interview with The New York Times that the European Union was not yet ready for a new round of tighter sanctions against Russia.