NATO Secretary General Jens Stoltenberg on Wednesday called on all parties to ensure the Organization for Security and Cooperation in Europe (OSCE)'s ceasefire monitoring in Ukraine and the implementation of the Minsk agreement.

He made the remarks at a joint press conference held at NATO's Supreme Headquarters Allied Powers Europe (SHAPE) headquarters in Mons, Belgium along with the Supreme Allied Commander Europe General Phillip Breedlove.

Stoltenberg said it is vital to make efforts to reach a peaceful solution to the conflict in Ukraine that the implementation of the ceasefire is possible to monitor.

He added that the OSCE monitors should have "the information, the freedom of movement and the security guarantees they require."

The ceasefire seems to be holding, but it "remains fragile," the NATO chief said.

Stoltenberg said NATO was adapting to a new security environment that is now more challenging than it has been for decades.

"Our answer, both to the challenges to the East and to the South, is that we are increasing our collective defense and we are now boosting our collective defense more than we have done since the end of the Cold War," he said.

Stoltenberg pointed out to NATO's exercise in the Black Sea where the United States is sending 3,000 troops and equipment to the Baltic region for training. Moreover, "in the south, we are preparing to hold this autumn our biggest exercise for many years, expected to include over 25,000 troops in this exercise," he said.

"We have seen the withdrawal of some heavy weapons, but it's unclear where it's now, what's the destination of the weapons that have been withdrawn," he said.

OSCE said Tuesday that the withdrawal process of heavy arms has shown signs of progress in Kiev.

The Ukrainian army and pro-independence insurgents started the withdrawal of their heavy weapons two weeks ago in line with the ceasefire deal reached in Minsk, the capital of Belarus, on Feb. 12, when both sides pledged a full artillery pullout.