"We're going to see how that all works out", Trump said. I have absolutely no doubt we did the right thing.
Erdogan's office said in a statement that the two leaders held a telephone conversation during which the Turkish leader stressed that the use of chemical weapons "is the greatest crime against humanity".
Earlier on Wednesday, after holding talks with the US secretary of state, Russian Foreign Minister Sergey Lavrov said some progress had been made on Syria, and a working group would be set up to examine the poor state of US-Russia ties. "We don't want a new Cold War".
Mr Trump, who strongly criticised the transatlantic alliance - which was formed in 1949 to counter the threat of the Soviet Union - during his election campaign, underlined the importance of the organisation during a press conference with Mr Stoltenberg. "It's no longer obsolete", he said, though he repeated previous calls by U.S. presidents for Nato's European members to increase their defence spending.
Lavrov said he emphasized the need for a wide-ranging OPCW probe during Wednesday's talks in Moscow with U.S. Secretary of State Rex Tillerson, suggesting that Western nations, Russian Federation and some regional powers could dispatch additional experts to join the investigation.
Speaking following the meetings, Mr Lavrov ruled out withdrawing his support for Syrian leader Assad, saying that "ousting a particular personality is not on our agenda". When Mr Tillerson's plane touched down in Moscow, it was not even clear whether President Vladimir Putin would deign to meet with him. The US blamed the Assad regime for the attack and accused Russian Federation of trying to cover it up.More news: Putin: US-Russia Relations Have 'Degraded' Since Trump Took Office
Assad, in an interview with Agence France-Presse, denied that any chemical weapons attack took place.
Trump, last week, had ordered missile strikes at a Syrian airbase after a chemical attack in the country's Idlib province killed 87 civilians, including many women and children.
"The perspective of the United States is that the recent chemical weapons attack carried out by Syria was planned, directed and executed by Syrian regime forces". "The world's two foremost nuclear powers can not have this relationship", he said.
Russian Federation and Syria claim the Khan Shaykhun victims were killed by toxic agents released from a rebel chemical arsenal hit by Syrian warplanes.
But he said Moscow and Washington had agreed to continue co-operating to try to find a political solution for Syria.
Mr Lavrov admitted relations between the two nations "are not the most calm" and some issues were "time bombs" inherited from the Obama administration.