Smoke plumes rise following reported airstrikes on the town of Khan Sheikhun in the southern countryside of the rebel-held Idlib province in Syria. (Anas Al-Dyab/AFP/Getty Images)

WHEN RUSSIA joined the Syrian regime of Bashar al-Assad in launching an offensive against the rebel-held province of Idlib in late April, U.N. officials warned that a humanitarian nightmare was in the offing: Three million civilians are squeezed into the territory along the Turkish border, many of them refugees from other parts of the country. As barrel bombs and artillery rounds rained down on hospitals, schools and food stores, the Trump administration reacted laconically. As late as last week, its Syrian envoy was repeating assurances offered by Moscow that “this is only a limited set of military operations against specific terrorists.”

Limited? Villages in southern Idlib have been hit by hundreds of airstrikes, according to reports from the region. Some two dozen hospitals have been attacked since April 30; according to the United Nations, 270,000 people have been driven toward the border with Turkey. Many are living in the open, “with some living under trees or plastic sheeting on bare patches of land,” according to U.N. official Ursula Mueller, who appeared last month before the Security Council.

On May 19, a village was reportedly attacked with the chemical weapon chlorine; though the State Department promised to investigate, there was no response. By Tuesday, according to the Syrian Observatory for Human Rights, at least 1,095 people had been killed in the offensive, including 338 civilians and 80 children.

The Assad regime does not regard the operation as “limited.” Its U.N. ambassador told the Security Council last week that it intended to retake the entire province, which is currently controlled by a mix of rebel groups backed by Turkey and extremists linked to al-Qaeda. The results could be catastrophic: U.N. and humanitarian officials predict a wave of up to 1 million refugees could push toward Turkey, which has sealed the border.

According to Turkish media, the government of Recep Tayyip Erdogan has responded to the offensive by mobilizing and arming its Syrian allies, who reportedly have been attacking the government’s forces with antitank missiles. But appeals by Mr. Erdogan to Russian President Vladi­mir Putin to stop the fighting and restore a cease-fire they negotiated last year have fallen on deaf ears.

The Russians and Syrians have paid no diplomatic price for their actions, even though the bombing of hospitals — by now a familiar part of their tactics — is a war crime. On Monday, Moscow’s ambassador squelched a proposed Security Council statement that would have expressed concern about “intensifying attacks on civilians and civilian objects, such as medical facilities and schools.”

So it was encouraging to see a tweet posted Sunday evening by President Trump: “Hearing word that Russia, Syria and to a lesser extent, Iran are bombing the hell out of Idlib Privince in Syria, and indiscriminately killing many innocent civilians. The World is watching this butchery. What is the purpose, what will it get you? STOP!”

Unfortunately, since then the bombing has only intensified. If Mr. Trump wants to stop the latest Syrian butchery, he will have to do more than tweet.