The US and Russian presidents have revealed the wide gulf between them over Syria, even as their foreign ministers were meeting in Moscow.
Donald Trump said Russia was backing “an animal” in Syria’s Bashar al-Assad.
Vladimir Putin said the US had violated the law in bombing a Syrian airbase and that the level of trust with the US had worsened since Mr Trump took office.
Rex Tillerson and Sergei Lavrov are trying to narrow the “areas of sharp difference” in their Moscow talks.
Tension in ties has risen since a suspected chemical attack on the rebel-held Syrian town of Khan Sheikhoun last week that left 89 people dead.
The US and its allies blamed the Syrian government, and the US fired 59 cruise missiles at the Shayrat airbase in response.
Syria denied carrying out the chemical attack and has the support of its main ally, Russia.
Speaking on the Fox Business Network, Mr Trump expressed his outrage at the chemical attack.
“You see these beautiful kids that are dead in their father’s arms, or you see kids gasping for life and you know… it’s over for them.”
He said he asked his defence minister for options and then “we hit them very hard”.
Referring to Mr Assad, he added: “Frankly, Putin is backing a person that’s truly an evil person… if Russia didn’t go in and back this animal, you wouldn’t have a problem right now.”
But Mr Trump also said: “Just so you understand, we’re not going into Syria.”
Speaking on Mir television, Mr Putin rejected allegations that Syria was behind the chemical attack, saying Syria had given up its chemical stockpile.
He said: “Where is the proof that Syrian troops used chemical weapons? There isn’t any.”
Referring to the US air strike, he said: “But there was a violation of international law. That is an obvious fact.”
And despite hopes that US-Russia ties would improve under a Trump presidency, Mr Putin said: “One could say that the level of trust on a working level, especially on the military level, has not improved, but rather has deteriorated.”
The presidents were speaking as Mr Tillerson and Mr Lavrov held talks.
Ahead of the meeting, Mr Lavrov said Russia had “a lot of questions regarding very ambiguous and contradictory ideas (…) coming from Washington”.
Mr Tillerson said he looked forward to a “candid” exchange so that the two countries could better define and narrow their differences.
Mr Tillerson has warned that Russia risks becoming irrelevant in the Middle East because of its support for Mr Assad.
However, the BBC’s Steve Rosenberg in Moscow says President Assad is Russia’s key military ally in the Middle East and Mr Tillerson may need to rethink his belief that he can weaken Moscow’s support for him.
Further evidence of division was shown when Russia’s foreign ministry said Moscow would later veto a draft resolution by the US, UK and France requiring the Syrian government to co-operate with an investigation into the chemical attack.
“In its current form this project is unacceptable for us,” deputy foreign minister Gennady Gatilov told the Interfax news agency. “We, of course, will not vote for it.”
President Putin has himself called for an independent UN investigation.