WASHINGTON — President Trump and Secretary of State Rex W. Tillerson sought on Wednesday to isolate President Vladimir V. Putin of Russia for backing the Syrian government in the wake of its lethal chemical weapons attack on civilians, and worked to build international pressure on Moscow to change course.

In Washington, Moscow and New York, the Trump administration publicly chastised Mr. Putin but privately worked to hash out increasingly bitter differences with him. At the same time, Mr. Trump embraced NATO — a military alliance he had previously derided as obsolete — as an effective and vital force for peace and security in a region where Russia has been an aggressive actor.

During his presidential campaign, and in his early days in office, Mr. Trump’s approach to foreign policy included speaking warmly of Mr. Putin and the prospects of a United States alliance with Russia. He had also questioned the usefulness of NATO, and the concept of an alliance for common defense to counterbalance Moscow’s belligerence.

In an interview that aired on Wednesday, Mr. Trump said that Mr. Putin was partly to blame for the conflict in Syria and denounced him for backing President Bashar al-Assad, whom he called an “animal.” Later at the White House, Mr. Trump said that Russia had likely known in advance of the Syrian government’s plan to unleash a nerve agent against its own people, and asserted that the United States’ relations with Moscow were at an “all-time low.”