German Chancellor Angela Merkel and Russian President Vladimir Putin agreed on Saturday to hold further talks about events in the Kerch Strait at the advisor level between Ukraine, Russia, France and Germany, her spokesman said.

Russia is resisting international calls to release three Ukrainian naval ships that its border patrols fired on and seized in the strait near Russian-annexed Crimea last weekend.

Moscow accused the 24 sailors of illegally crossing the Russian border.

Merkel voiced her concern about the situation when she met Putin on the sidelines of the G20 summit in Buenos Aires, said the spokesman, Steffen Seibert.

‘The chancellor once again made clear her concern about the escalation in the Kerch Strait and her advocacy for freedom of the seas in the Sea of Azov,’ Seibert said.

Ukraine on Friday filed a complaint against Russia in the European Court of Human Rights over the incident, which took place in the strait that separates the Sea of ??Azov from the Black Sea.

Merkel and Putin agreed to continue discussing the dispute at the advisor level in the four-way Normandy format that has sought to ease tensions between Ukraine and Russia.

Senior officials from the four countries gathered in Berlin last week for a meeting planned before the Ukrainian ships were seized. It was not immediately clear when follow-up discussions would take place.

Merkel and Putin also discussed the situation in Syria, and agreed more effort should be made to work towards the implementation of agreements by Germany, France, Turkey and Russia at a summit in Istanbul in October, Seibert said.