Mr. Trump has been critical of Russia and its president, Vladimir V. Putin, for supporting the Syrian regime, led by Bashar al-Assad, believed to be behind the suspected chemical weapons attack on April 7 that has left dozens dead.

The attack on Saturday in the Damascus suburb of Douma has not been confirmed to be the result of a chemical weapon.

Defense Secretary Jim Mattis said on Wednesday that the United States is still assessing the intelligence on the suspected chemical attack, but that military planning was proceeding.

“We stand ready to provide military options if they’re appropriate, as the president determined,” he said.

The World Health Organization said on Wednesday that there were reports of about 500 people in the Damascus suburb of Douma who have symptoms similar to people exposed to toxic chemicals. It said about 70 people had died while taking shelter in basements and 43 of them had signs of being exposed to “highly toxic chemicals.”

Mr. Trump’s comments about poor relations with Russia echoed what the Russian foreign minister, Sergey V. Lavrov, said recently in response to the wave of diplomatic expulsions of Russians from the United States and other countries, according to a Reuters report. The expulsions were a coordinated response to the poisoning in Britain of a former Russian spy and his daughter. Since then, analysts have said the Balkans could become a battleground for a new Cold War.