Russia has detained more than 150 foreign vessels heading for Ukrainian ports in the Sea of Azov, Oleh Slobodian, assistant to Ukraine's State Border Guard Service chief, has said.

"I don't have exact data, but it seems that more than 150 vessels heading for Ukrainian ports have been detained. The trend is stable: Russian border guards continue to inspect those vessels that are heading to Ukrainian ports in the Sea of Azov," Slobodian told the 112 Ukraine TV channel on Tuesday.

He added that vessels heading toward Russian ports are not detained.

"They [the Russians] spend a lot of time issuing permits for crossing the Kerch Strait to the vessels heading to Ukrainian ports. The ships have to wait several days before crossing. Sometimes, Russian vessels heading to Russian ports are not delayed at all," he said.

Slobodian said inspects of ships heading to Ukrainian ports are usually carried out when they are in areas not accessible to Ukrainian sea patrol boats.

He said Ukrainian vessels heading for Ukrainian ports are stopped in remote areas not accessible to Ukrainian border guards.

"The vessels stopped are mostly those heading to Ukrainian ports. They are stopped in the most remote areas. Their tactics, again, change. The Russians feel free to act where our [border service] vessels are not present," Slobodian said.

As earlier reported, Ukraine's Infrastructure Ministry said that Russia detained 93 vessels from April 29 through July 9, 2018 without providing any reasons.

Ukraine's Infrastructure Minister Volodymyr Omelyan said Russia is deliberately creating impediments for vessels heading to Ukrainian ports in the Sea of Azov and called on Ukraine to impose tit for tat sanctions as well as initiating international sanctions against Russia.

Ukraine's President Petro Poroshenko on July 17 tasked Ukraine's Defense Ministry to take measures in connection with the illegal stopping and checks of Ukrainian and foreign vessels heading to ports in the Sea of Azov.