The ships, Admiral de Ribas and Mount Baker, left port days after Russia suspended its participation in an agreement that allowed vital grain shipments to pass through the Black Sea.

Twelve ships were due to leave Ukrainian ports on Monday and four were to head to the country, according to the Joint Coordination Center (JCC) that has been overseeing the agreement brokered by Turkey and the UN.

Russia has notified the UN and Turkey of its plans to suspend the agreement for an indefinite period of time after Moscow accused Ukraine of a "massive" drone attack on its Black Sea Fleet in Crimea.

The July deal to unlock grain exports signed between warring nations Russia and Ukraine is critical to easing the global food crisis caused by the conflict.

"Civilian cargo ships can never be a military target or held hostage. The food must flow," Amir M. Abdulla, UN Coordinator for the Black Sea Grain Initiative, tweeted on Monday.

The agreement, which established a safety corridor through which vessels could travel to Istanbul for inspections, had already allowed more than nine million tonnes of Ukrainian grain to be exported and was due to be renewed on November 19.