Iran has accused the United States of a "dangerous escalation" in Syria after Washington warned President Bashar al-Assad may be preparing another chemical attack for which he would pay a "heavy price."
"Another dangerous U.S. escalation in Syria on fake pretext," Iranian Foreign Minister Mohammad Javad Zarif tweeted on June 27, adding it "will only serve [the Islamic State extremist group] precisely when it's being wiped out by Iraqi and Syrian people."
Iran's accusation came as French President Emmanuel Macron joined with U.S. President Donald Trump in vowing a joint response against any new Syrian chemical attack in a telephone call on June 27.
Iran is Assad's closest ally along with Russia, which also denounced what the Kremlin called Washington's "unacceptable threats." Iran provides Assad with troops while Russia provides air support in his war against rebels and IS extremists.
The Pentagon said earlier on June 27 that U.S. intelligence had identified what appear to be preparations by Syrian forces for a chemical attack, seen at the same Shayrat airfield used to launch a previous attack that killed dozens of civilians in April.
The Pentagon and White House warned that Syria will pay a "heavy price" for another attack. The United States launched cruise-missile strikes on Shayrat following the April attack, in what was the first U.S. military action against Syria in its 6-year-old civil war.
U.S. forces have increased their presence in Syria to about 1,000 in recent months as the Pentagon steps up its campaign against IS, which is being targeted by U.S.-backed rebel forces in its stronghold of Raqqa.
But the U.S. forces have increasingly clashed as well with Syria and Iran's forces operating in the same battle space.
The United States has shot down several Iranian drones, bombed Iranian-backed militia allied with Syria, and shot down a Syrian jet that the Pentagon said was preparing to bomb U.S.-backed rebel troops.
The White House made it clear on June 27 that the latest warning against Syrian chemical attacks was not aimed only at Assad but targeted his allies as well.
"The goal is at this point not just to send Assad a message, but to send Russia and Iran a message," U.S. Ambassador to the United Nations Nikki Haley said.
Assad did not respond to the U.S. warnings as he toured an air base in western Syria that hosts Russian aircraft and troops on June 27. Accompanied by the Russian Army's chief of staff, General Valery Gerasimov, Assad climbed into the cockpit of a Russian Su-35 fighter jet.
"The Syrian people will not forget the support of their Russian brothers," Assad wrote in the visitors' book at the Hmeimim base.