“I don’t know what the hell happened to Putin,’’ President Trump told reporters on Sunday afternoon, just before boarding Air Force One for a short trip from his golf club in New Jersey to Washington. Hours later, he posted about the Russian leader, saying, “He has gone absolutely CRAZY.”

Mr. Trump’s rare criticism of President Vladimir V. Putin of Russia came after a weekend of the largest bombardment of Ukrainian cities over the past three years, mostly aimed at civilian targets, from residential areas in Kyiv to university dormitories. The Russian attacks also happened only days after Mr. Trump had what he described publicly as an “excellent” two-hour phone call with Mr. Putin that Mr. Trump promised would immediately lead to direct peace negotiations.

Mr. Trump has long said he enjoys a “good relationship” with Mr. Putin, and it was not the first time he expressed shock that the Russian president was unleashing attacks on Ukrainian civilians. A month ago Mr. Trump wrote “Vladimir, STOP” as a barrage of missiles and drones hit Ukraine, including crowded playgrounds. But Mr. Trump has never linked the attacks with his own decision, reaffirmed last week, to refuse to join the Europeans in new financial sanctions on Russia, or to offer new arms and help to the Ukrainians.

The result is a strategic void in which Mr. Trump complains about Russian’s continued killing but so far has been unwilling to make Mr. Putin pay even a modest price.

The pattern is a familiar one, several outside experts and former government officials said. Mr. Trump signals he is pulling back from a conflict he often describes as Europe’s war, then expresses shock that Mr. Putin responds with a familiar list of demands that amount to a Ukrainian surrender, followed by accelerating attacks. Mr. Trump episodically insists he is “absolutely” considering sanctions, including on Sunday.