WASHINGTON -- After U.S. President Donald Trump's election victory, the United States and Russia appeared headed toward their smoothest ties in decades. Not anymore.

The former Cold War adversaries are once again spiraling into confrontation, punctuated by a U.S. attack on a military base controlled by Syrian President Bashar Assad, Russia's client. No longer optimistic about a "reset" in relations, the U.S. and Russia openly bashed each other Friday, trading caustic accusations about who violated international law.

"That's it. The last remaining election fog has lifted," Russian Prime Minister Dmitry Medvedev wrote on Facebook on Friday, declaring U.S.-Russian relations "completely ruined." He said Washington came dangerously close to "a military clash" with nuclear-armed Moscow by firing 59 cruise missiles on the Shayrat air base. Trump said Assad's forces launched a gruesome chemical weapons attack from the site earlier in the week.

Trump's intervention, designed to punish Assad, was the clearest demonstration of his willingness to challenge Russian President Vladimir Putin -- and in a way no American leader has in a long time.

Trump's praise for Putin, questionable assertions about Russia's military activity in Ukraine and Syria, and insistence on a new relationship with Moscow had generated the perception that the billionaire businessman wouldn't cross the former KGB agent. It's a perception that gained added currency as various U.S. investigations gained steam into possible collusion on election meddling between Trump's presidential campaign and Russian intelligence.

Now the question is if Putin will feel compelled to prove he can't be crossed with impunity.

Hours after the airstrikes, Russia announced it was severing a hotline the two countries have used since 2015 to ensure their aircraft don't accidentally clash in Syria's crowded skies. By midday Friday, the U.S. insisted that Russia would keep the "deconfliction" channel open. Russia then insisted the line would be suspended midnight Saturday in Moscow.

But Trump's administration shot back, as senior U.S. military officials said they were investigating whether Russia was complicit in the Syrian military's use of a sarin-like nerve gas, possibly by providing drone surveillance and helping Assad's forces try to cover up the attack. At the U.N. Security Council, U.S. Ambassador Nikki Haley mocked Moscow for failing to rid Syria of chemical weapons under a 2013 deal.

"It could be that the Assad regime is playing the Russians for fools," Haley said.

Less than three months into Trump's administration, prospects have all but evaporated for collaboration with Russia on fighting the Islamic State group, reducing arms stockpiles and lowering tensions in Eastern Europe.

A key test of whether the relationship can be salvaged comes next week when Secretary of State Rex Tillerson becomes the first Trump Cabinet member to visit Russia. Tillerson may get an audience with Putin himself.

Despite the breakdown over Syria, where Russia has a significant military presence, U.S. officials insisted Tillerson's highly anticipated trip was still on. As top U.S. and Russian representatives raged against each other in public, their planning for the Moscow trip has continued without issue, said the officials, who weren't authorized to discuss private diplomatic conversations and requested anonymity.

For Tillerson, the trip is even more delicate than before: He must find a way to show the U.S. can stand up to Russia and safeguard elements of co-operation at the same time. He must also be prepared to deal with the notoriously unpredictable Putin, known for making guests feel uncomfortable when he wants to express displeasure.

"Let him come and tell us what they have been up to today," Maria Zakharova, the Russian Foreign Ministry spokeswoman, told Russian television. "We will tell them everything that we think on this score."

If Putin is looking for a way to even the score, it might not be in Syria, said Julianne Smith, a former National Security Council and Defence Department official now at the Center for a New American Security. Faced with challenges to his country's dignity, she said, Putin always thinks in "asymmetric terms."

"We should be watching eastern Ukraine, we should be watching for a cyberattack, another drip-drip-drip of WikiLeaks," she said. "There's all sorts of things they can do."

Relations with Russia have deteriorated since its 2014 annexation of Ukraine's Crimea region and assistance for pro-Russian separatists in eastern Ukraine. The Obama administration and Western countries slapped Moscow with severe economic sanctions that failed to get Russia to change course.

Central to the FBI and congressional investigations into Russia's election meddling are concerns that Trump, through his associates, may have signalled to Russia that he would lift the sanctions after taking office. Trump has denied that claim. It now appears unlikely the sanctions could be lifted anytime soon.

In Syria, at least, the two countries have strenuously avoided direct confrontation, even as their militaries support opposing sides in a six-year civil war. To Washington's relief, Russia said none of its troops were killed in Thursday night's strikes. Syria's military said seven of its own troops were killed.

But in response, Russia said it would strengthen Syrian air defences, already bolstered by Russia over the last year. Doing so raises the likelihood of a more serious military confrontation if Assad uses chemical weapons again and Trump feels compelled to respond.

AP Diplomatic Writer Matthew Lee in Washington and AP writer Vladimir Isachenkov in Moscow contributed to this report