Senior US officials have warned the Russian and Syrian governments against chemical weapons use in Syria as forces allied with President Bashar Al Assad prepare for an offensive on the rebel stronghold of Idlib.
The United States "will respond to any verified chemical weapons use in Idlib or elsewhere in Syria ... in a swift and appropriate manner," State Department spokeswoman Heather Nauert said in Washington on Tuesday.
Ms Nauert said senior American officials engaged with their Russian counterparts to "to make this point very clear to Damascus", noting that the use of chemical weapons "will not be tolerated".
Secretary of State Mike Pompeo told Russian Foreign Minister Sergey Lavrov last week that Moscow – a military ally of Mr Al Assad – would be held responsible.
The Syrian government is gearing up for an expected offensive in Idlib province, which is home to nearly three million people and has a large Al Qaeda presence in addition to Syrian rebel groups.
US President Donald Trump has twice carried out air strikes in Syria in response to apparent chemical weapons attacks there. Mr Trump said the strikes were intended to deter the Syrian regime from launching chemical weapons attacks again.
At the Pentagon, Defence Secretary Jim Mattis referred to those two air strikes but offered no further information on how the US was responding to the situation, other than to cite the State Department's "recent active communication with Russia to enlist them in preventing this".
In April, the US, France and Britain launched military strikes in Syria in a bid to punish Mr Al Assad for an apparent attack using chlorine against civilians in the Damascus suburb of Douma. And in 2017 Mr Trump authorised a barrage of Tomahawk cruise missiles to hit a single Syrian airfield in retaliation for what the US said was the Syrian president's use of sarin gas against civilians.
Mr Al Assad has repeatedly denied his government has used chemical weapons.
Russia has accused Syrian rebels of preparing chemical attacks, which Moscow says the West will use to justify a strike against Syrian government forces.