BRUSSELS—U.S. and Ukrainian officials called Monday for tougher measures against Russia over its seizing of three Ukrainian naval vessels in the Black Sea. The incident late last month caused Kiev to declare martial law but drew little more than condemnations from Western capitals.

Speaking to reporters Monday afternoon, Kurt Volker, the U.S. special representative for Ukraine, advocated, at a minimum, for targeted sanctions against certain Russian officials, while Ukraine Prime Minister Volodymyr Groysman backed combining those measures with broader economic sanctions.

European Union leaders last week condemned Russia’s use of military force against Ukrainian vessels. They also demanded that Moscow release 24 Ukrainian sailors and restore the right of passage through the Kerch Strait, which connects the Black Sea to the Sea of Azov.

However, the EU, which is set to extend longstanding economic sanctions against Russia by another six months, has so far taken no action on the incident, which U.S. officials described as a reckless escalation of tensions between Ukraine and Russia. The EU imposed the sanctions in 2014 after Russia annexed Crimea and Sebastopol, but deep divisions within the bloc over its Russia policy have limited further significant pressure.

EU powerhouses Germany and France have hesitated to increase European pressure on Russia over the incident, with European diplomats saying their focus was on the release of Ukraine’s sailors and the easing of tensions. EU foreign-policy chief Federica Mogherini said Monday the bloc was looking at fresh ways to help Ukraine’s economy after disbursing half a billion euros of new loans to Kiev last week.