Russian Foreign Minister Sergei Lavrov said that Russian aid workers had visited the area and found no evidence that chemical weapons were used.
Russia is Syria's main military partner and ally and has protected Syrian President Bashar al-Assad both militarily and diplomatically for years.
Russia helped the United States negotiate an agreement with Assad five years ago that was supposed to rid the country of chemical weapons, and the United States and other nations have held Russia responsible for allowing Assad to retain and use these weapons.
Asked by a reporter if he had any doubt who was behind the attack, Trump said: "To me there's not much a doubt, but the generals will figure it out."
He said "nothing is off the table" when asked if US military action is an option.
Trump said it has been "very hard to get people in" to determine what happened, which he said was a sign of Syria's culpability.
Trump's comments came as the UN Security Council was preparing to meet in an emergency session today to discuss the suspected chemical attack.
The Security Council meeting is shaping up to be a rematch of previous debates on civilian casualties in Syria. Though the council unanimously approved a ceasefire resolution in February, no truce has ever stuck.
Russia voted for the ceasefire but has repeatedly used its veto power as a permanent member to block any measure that threatens a Syrian offensive against rebels, largely to protect the Government of Assad.
Russia wants to preserve access to its only naval base in the Middle East and preserve its influence in the region through Assad.
