BERLIN (Reuters) – German Defense Minister Ursula von der Leyen urged Russia on Tuesday to release Ukrainian sailors captured at the weekend and said both parties needed to show they were trying to calm the situation.

Russia seized three Ukrainian naval ships and took their crew prisoner at the weekend, prompting Ukraine’s President Petro Poroshenko to say there was now an “extremely serious” threat of a Russian land invasion.

Von der Leyen told the Berlin Security Conference that the circumstances were not yet fully understood, but it was critical to avoid the situation spiraling out of control.

“This is about the question of respect for territorial integrity and about the will not to let disputes escalate,” she said.

“The Kremlin has violated these rules and principles in the past years. In this current situation, both sides, Russia and Ukraine, must show that they are actively contributing to de-escalation,” von der Leyen added.

She said the seized ships and sailors must be released. “Ukraine must provide evidence about the exact events. Russia must ensure free passage through waterways and it should not act in an disproportionate way,” she added.

German Chancellor Angela Merkel spoke to Russian President Vladimir Putin on Monday and stressed the need for de-escalation and dialogue, her spokesman said on Tuesday.