Trump's repeated claims that he could mend relations between Washington and Moscow has fuelled accusations that he secretly colluded with Russian Federation to win the USA presidential election previous year.
The stark language is the most blunt terms Trump has used to describe the Syrian president who is accused of war crimes in an attempt to hold onto power during a prolonged civil war.
Syria denied any use of chemical weapons and Russian Federation backed its claim.
"We have information that a similar provocation is being prepared... in other parts of Syria including in the southern Damascus suburbs where they are planning to again plant some substance and accuse the Syrian authorities of using chemical weapons", he said on Tuesday.
"Despite the quantity of existing problems... there are considerable prospects for joint work", Russian Foreign Minister Sergey Lavrov told a news conference after talks with Tillerson and Russian President Vladimir Putin.
British Foreign Secretary Boris Johnson said he was "dismayed" by Russian Federation vetoing Wednesday a United Nations draft resolution on the suspected chemical attack in Syria.
Despite rolling out a red carpet for the U.S. Secretary of State upon his arrival in Moscow, the Kremlin delivered a frosty reception to Rex Tillerson in a face-off over Syria. U.S. -Russia relations were supposed to improve under the Trump administration.
Lavrov said both sides were set on leading an "uncompromising fight against worldwide terrorism".
Lavrov is also subtly mocking Tillerson for the fact that top U.S. State Department positions are unfilled.
"The U.S. isn't serious in looking for any kind of a political solution", Assad said.
Trump said during the meeting that North Atlantic Treaty Organisation is "no longer obsolete", reversing his election campaign stance on the bloc, which had alarmed some of America's closest allies in Europe. "I think it's very bad for Russia", Trump said in an interview with Fox Business on Wednesday. "How do you know that?" he said.
Russian Foreign Minister Sergei Lavrov, center, stands in the middle as Syrian Foreign Minister Walid Muallem, left, and Iranian Foreign Minister Mohammad Javad Zarif shake hands after a shared press conference following their talks focused on Syria in Moscow, Russia, April 14, 2017.