Russia opened fire on the three Ukrainian vessels and then seized them and their crew on Sunday near Crimea, which Russia annexed from Ukraine in 2014.
Russia must release the Ukrainian boats and sailors it captured on Sunday, British Prime Minister Theresa May said on Thursday, adding that Britain would continue to push for "appropriate sanctions."
Russia opened fire on the three Ukrainian vessels and then seized them and their crew on Sunday near Crimea, which Russia annexed from Ukraine in 2014. Moscow and Kiev have tried to pin the blame on each other for the incident.
"This is part of a pattern of Russian behaviour. They should release those ships and sailors, they should de-escalate the situation,” May told reporters on the flight to the G20 summit in Argentina, which Russian President Vladimir Putin is also due to attend.
"We have always been at the forefront in the EU of calling for sanctions on Russia in relation to its behaviour,” she added. “We will continue to push for what we consider to be appropriate sanctions on Russia.”
Earlier this week several senior European politicians raised the possibility of new sanctions against Russia to punish it for capturing the boats.