WASHINGTON (NYTIMES) - United States President Donald Trump and Secretary of State Rex Tillerson sought on Wednesday (April 12) to isolate President Vladimir 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 Putin but privately worked to hash out increasingly bitter differences with him. At the same time, 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, Trump's approach to foreign policy included speaking warmly of Putin and the prospects of a US alliance with Russia. He has also questioned the usefulness of Nato and the concept of an alliance for common defence to counterbalance Moscow's belligerence.

In an interview that aired Wednesday, Trump said that Putin was partly to blame for the conflict in Syria and denounced him for backing President Bashar Assad, whom he called an "animal."

Later at the White House, 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."

In Moscow, Tillerson came away from a two-hour meeting with Putin - the first such face-to-face session of the Trump administration - without reaching agreement on facts involving the chemical weapons assault in Syria or Russian interference in the US election. And sharply diverging from the meeting of the minds between the United States and Russia that Trump frequently aspired to when he was campaigning, there was no visible warming of the relationship.

"There is a low level of trust between our countries," Tillerson told reporters at a joint news conference with his Russian counterpart, Foreign Minister Sergey Lavrov. "The world's two foremost nuclear powers cannot have this kind of relationship."

The most immediate casualty of the clash was Russia's decision last week to suspend a communication channel, set up in 2015, to share information about US and Russian air operations over Syria to avoid possible conflict. Lavrov said Wednesday that "we're willing to put it back into force" if Washington and Moscow can resolve their differences.

Further punctuating the Syria dispute, Russia on Wednesday vetoed a Western-backed resolution at the United Nations Security Council that condemned the chemical weapons attack. It was the eighth time in the 6-year-old Syrian civil war that Russia, one of the five permanent Security Council members, had used its veto power to shield the government in Damascus. But in a possible sign of Russia's isolation on the chemical weapons issue, China, the permanent member that usually votes with Russia on Syria resolutions, abstained.

The vote came the day after Trump spoke by phone to President Xi Jinping of China, whom he hosted last week at a summit at his Mar-a-Lago retreat in Palm Beach, Florida. White House officials said they credited the relationship between the two leaders that was forged during the visit, and the conversation Tuesday evening, with helping to influence China's vote.

The day began with harsh words from Trump toward Putin, whom he had once praised effusively.

"I really think there's going to be a lot of pressure on Russia to make sure that peace happens, because frankly, if Russia didn't go in and back this animal, we wouldn't have a problem right now," Trump said in an interview with Fox Business Network, referring to Assad.

"Putin is backing a person that's truly an evil person, and I think it's very bad for Russia. I think it's very bad for mankind. It's very bad for this world."

Later, after a meeting at the White House with Jens Stoltenberg, the Nato secretary-general, Trump went out of his way to praise the military institution, which he called a "great alliance," and to express disappointment with Russia.

Asked whether it was possible that Syrian forces could have launched the chemical attack without Russia's knowledge, Trump said: "It's certainly possible; I think it's probably unlikely." "I would like to think that they didn't know, but certainly they could have. They were there," Trump said of the Russians during a 30-minute news conference at the White House.

Even as they have intensified their criticism of Russia for backing Assad, other senior Trump administration officials, including Tillerson and Jim Mattis, the secretary of defense, have been careful to say there is no consensus that Moscow had foreknowledge that the Assad government planned to launch a chemical assault.

"Right now, we're not getting along with Russia at all - we may be at an all-time low in terms of a relationship with Russia," Trump said Wednesday. Still, he held out hope that the United States and Russia could come to terms, suggesting that Tillerson's talks with Putin had gone better than expected.

A quick détente seemed a remote possibility, given the level of tension surrounding the aftermath of the Syrian chemical weapons attack. On Tuesday, the White House accused Putin's government of covering up evidence that Assad had been responsible for the sarin assault, which was launched from a base where Russian troops are operating.

Putin shot back that the charge was fabricated and accused the administration of Trump, who US intelligence agencies believe benefitted during the election campaign from Russian cyberattacks intended to embarrass his Democratic rival, of fabricating the evidence to create a fake confrontation.

Amid the rift with Russia, Trump made a striking reversal on Nato, saying the alliance had transformed into an effective one since he took office.

"I said it was obsolete; it's no longer obsolete," Trump said, standing beside Stoltenberg.

Trump attributed his change of heart to unspecified transformations within Nato, which he said were a direct response to criticism he had levelled that the alliance was not doing enough to combat terrorism.

"I complained about that a long time ago," Trump said, "and they changed." It was not clear what the president was referring to; Nato forces have been fighting alongside the United States in Afghanistan for more than a decade, an effort focused on combating terrorist groups including the Taliban.

Still, the turnabout drew praise from some lawmakers who had been concerned with Trump's previous stance.

"Without Nato, the Soviet Union would be quarterbacking half of Europe today and Putin knows it," said Nebraska Republican Senator Ben Sasse. "Nato is the most successful military alliance in human history. This was the right decision."