The United States has accused Syria and Russian Federation of using tear gas against civilians in an alleged "false flag" attack blamed on rebels and jihadis, which came around the same time the Trump administration received criticism for using the substance against asylum seekers at the US border.
The US state department said the goal was to "undermine confidence" in the ceasefire in Idlib province.
In a statement Friday, the US State Department said the Syrian government and its ally, Russia, "falsely accused the opposition and extremist groups of conducting a chlorine attack" in northwestern Aleppo on 24 November. The Russian military did not rule out that the White Helmets non-governmental organisation could be behind the attack, in which more than 70 civilians were injured.
Officials with Syrian rebel groups denied the allegations, however, and said their forces did not possess chemical weapons.
"We caution Russian Federation and the regime against tampering with the suspected attack site and urge them to secure the safety of impartial, independent inspectors so that those responsible can be held accountable", the US State Department said.
Palladino said that the United States is concerned that Syrian officials have maintained control of the site to "potentially fabricate samples and contaminate the site before a proper investigation of it by the Organization for the Prohibition of Chemical Weapons".
Parts of the Aleppo region, as well as the neighbouring provinces of Hama and Idlib, are controlled by Turkish-backed rebels and jihadists. Syrian state media also reported the account, saying that about 100 Syrians were hospitalized with breathing difficulties.
Western powers and the United Nations have accused government forces of using chemical and nerve agents on numerous occasions since the start of the civil war.