An advisor to the chief of the Security Service of Ukraine (SBU), Markiyan Lubkivsky, has said that Russia is trying to block Mariupol from the sea and is preparing for active hostilities.

"Today we see and document a buildup in Russia's military presence in the temporarily occupied territories, troops are being redeployed, and positions are being fortified. There are all symptoms that Russia... is planning or preparing for active hostilities," he said on the Shuster Live program late on Wednesday.

However, he said that it was unnecessary to succumb to panic and "it's necessary to stay calm."

"Today we are in a state of enhanced readiness... May subversive groups are now being liquidated and even destroyed," he said.

Lubkivsky also said that "the situation near Mariupol is tense."

"In our opinion, Russians mine the waters of Mariupol. The issue concerns the explosion of the so-called bottom mine. It is not excluded that it could have hit a fishing schooner," he said, speaking about the explosion in the sea near Mariupol.

"This is one of the symptoms that Russia is trying to block Mariupol from the sea," he said.