Mr. Putin’s nuclear threat came as much of the world declared him an international pariah. He is among a small number of leaders to be hit by personal sanctions. That club includes Mr. al-Assad of Syria, North Korea’s Kim Jong Un and Venezuela’s Nicolás Maduro.

Now evidence is mounting that even some allies may be moving away from him. Turkey is weighing a request from Ukraine to block Russian warships from entering the Black Sea through a strategic chokepoint. Ibrahim Kalin, a spokesman and top aide to President Recep Tayyip Erdogan, tweeted Sunday that his country would “continue our efforts to help the people of Ukraine and end bloodshed in this unjust and unlawful war.” Azerbaijan, a former Soviet republic and ally of Russia, has sent humanitarian aid to Ukraine.

Mr. Putin will face added pressure from ordinary Russians who are already being hit by sanctions. Long lines at ATMs over the weekend prompted the central bank to say the Russian financial system was stable. The ruble and stocks are expected to tumble in Moscow on Monday as sanctions hit those markets. Prominent Russians, including some oligarchs, have criticized the invasion while hundreds of Russians have joined street protests violently suppressed by riot police in recent days.