The White House said on Monday it appears the Syrian government is preparing for another chemical weapons attack and it warned Syrian President Bashar al-Assad he and his military would “pay a heavy price” if it conducts such an attack.

The White House statement said preparations by Syria were similar to those undertaken before an April 4 chemical attack that killed dozens of civilians and prompted President Donald Trump to order a cruise missile strike on a Syrian air base.

Trump ordered the strike on the Shayrat airfield in Syria in April in reaction to what Washington said was a poison gas attack by Assad’s government that killed at least 70 people in rebel-held territory. Syria denied it carried out the attack.

READ MORE: Chemical weapons watchdog ready to travel to Syria to investigate attack if safety assured

The strike was the toughest direct U.S. action yet in Syria’s six-year-old civil war, raising the risk of confrontation with Russia and Iran, Assad’s two main military backers.

U.S. officials at the time called the intervention a “one-off” intended to deter future chemical weapons attacks and not an expansion of the U.S. role in the Syrian war.

The United States has taken a series of actions over the past three months demonstrating its willingness to carry out strikes, mostly in self-defense, against Syrian government forces and their backers, including Iran.

WATCH: White House accuses Russia of covering up Syrian regime’s role in chemical attack

4:11 White House accuses Russia of covering up Syrian regime’s role in chemical attack

White House accuses Russia of covering up Syrian regime’s role in chemical attack

The United States ambassador to the United Nations Nikki Haley said on Twitter: “Any further attacks done to the people of Syria will be blamed on Assad, but also on Russia and Iran who support him killing his own people.”

Any further attacks done to the people of Syria will be blamed on Asaad, but also on Russia & Iran who support him killing his own people. — Nikki Haley (@nikkihaley) June 27, 2017

Since the April military strike, Washington has repeatedly struck Iranian-backed militia and even shot down a drone threatening U.S.-led coalition forces. The U.S. military also shot down a Syrian jet earlier this month.

READ MORE: Canada announces sanctions against Assad regime following chemical attacks in Syria

Trump has also ordered stepped-up military operations against the Islamic State militant group and delegated more authority to his generals.

The White House has provided no immediate evidence to back up its claims.

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