The attack, which US officials believe took place with the knowledge of high-ranking Syrian government officials, prompted Trump to launch the USA military's first direct assault on an Assad-linked target since Syria's war began six years ago. The US has insisted Syria secretly kept chemical weapons from the 2013 disarmament and used those, attacking the Syrian airbase in retaliation.
In 2014, the USA and Russian Federation brokered a deal to destroy all of Syria's chemical weapons as punishment for a chemical weapons attack that killed at least 1,300 people in Damascus on 21 August 2013.
The trio was meeting after an April 4 chemical incident in Syria's Khan Sheikhoun in Idlib Governorate and the subsequent U.S. strike at an airbase in Homs Governorate.
Russian Federation contends the chemicals belonged to terrorists.
In October 2016, Assad claimed that a photograph taken in Aleppo of 5-year-old Omran Daqneesh in the aftermath of a airstrike from Syrian or Russian planes - and which became a symbol of the conflict in Syria - had been manipulated or forged. Moscow has claimed the suspected attack was the work of a rebel chemical weapons depot.
An investigation is now being requested by the OPCW. Earlier testing by Turkish authorities had also said the chemical used on April 4 was sarin.
The Kremlin in a statement confirmed that Putin and Erdogan had spoken "in favor of conducting an objective and full global investigation as soon as possible". More than 80 people lost their lives and scientists have found samples of sarin, a lethal nerve agent with no smell, taste or color.
The OPCW's Fact Finding Mission for Syria is conducting an investigation and is expected to report its findings in three weeks.
Moscow vetoed a Western draft United Nations resolution Wednesday, saying it failed to mention the need to inspect the area of the attack.
President Trump ordered an airstrike on a Syrian air base the US believes was the launch point for the attack, which reported involved sarin gas, in retaliation.
CNN interviewed victims of the attack, who fled to nearby Turkey, who recounted the horrific aftermath of a gas attack, saying the town of Khan Sheikhoun was struck by regime warplanes.
The strains in ties were clear as US Secretary of State Rex Tillerson received a frosty reception Wednesday on a visit to Moscow, where he said there was now "a low level of trust" between the two countries.
"We are deeply anxious by our partners in the UN Security Council trying to evade an honest investigation", Lavrov said.