But a report issued by Andrii Klymenko, the head of an independent research group in Kiev that monitors maritime traffic, suggested that the Ukrainian border service had inflated the number of stranded ships.

The problem, he said, is no longer so much that Russia is blocking passage, but that shipowners, unnerved by previous aggression by Russia and alarm over its future intentions, have stopped sending vessels to Mariupol. He predicted that in coming days, the city’s port would be mostly empty “because of the fears of shipowners.”

Such fears have highlighted how easy it is for Russia to squeeze Mariupol and Ukraine as a whole by dialing up and down pressure. It eases off when it wants to head off calls abroad for sanctions and assert plausible deniability, while leaving such a cloud of uncertainty that nobody can be sure what the risks are.

It has done this throughout its more than four-year-long conflict with Ukraine, sending troops into eastern Ukraine whenever its local proxies lose ground to Ukrainian forces and then pulling back just enough whenever Western outrage threatens to bubble over.

Ukraine has itself often played into Russia’s hands by engaging in sometimes wild exaggeration that only increases uncertainty and alarm. The infrastructure minister, for example, repeatedly compared Russia’s president, Vladimir V. Putin, to Hitler during his visit to Mariupol and warned that Russia was poised to invade not only Ukraine, but also Western Europe. Such hyperbole gives traction to Russian propaganda that depicts Ukraine as untrustworthy and hellbent on war.

Russia’s attack on three small Ukrainian naval vessels near the Kerch Strait, which Mr. Putin dismissed as a minor “border incident,” took place in international waters, according to an analysis of Russian-provided maritime coordinates by the investigative group, Bellingcat. It was the first time that Russia has clashed openly with Ukraine’s military, instead of through local proxies and Russian soldiers in unmarked uniforms, known as “little green men.”

