Photo: The Canadian Press

The Syrian government on Tuesday dismissed White House allegations that it was preparing a new chemical weapons attack.

The Trump administration said Tuesday it has detected "active preparations" by Syria for a chemical attack and threatened to retaliate — warning of action that could plunge America deeper into a civil war alongside the fight against Islamic State militants.

The chemical threat and sudden White House warning illustrate the challenging complexities of the fighting in Syria, a country whose territory was used by IS to march into Iraq in 2014 and prompt a U.S. return to the Middle East's battlefield. Washington now has more than 5,000 troops in Iraq and about 1,000 in Syria.

President Donald Trump has said he won't stand for Syria's use of chemical weapons, which are banned under international law and are particularly worrisome in the Arab country because they could fall into extremists' hands.

The Pentagon said the preparations detected by the U.S. occurred at the same air base where Syrian aircraft embarked on a sarin gas strike on April 4, killing almost 90 people. Days later, Trump ordered a cruise missile attack against the base in retaliation.

The Syrian government has denied it ever used banned chemicals, and it rejected Washington's latest allegation Tuesday.

Syria's two main allies, Russia and Iran, joined in bashing Washington. Iran's foreign minister called the U.S. threat a "dangerous escalation." A senior Russian lawmaker accused the U.S. of a "provocation."

It was unclear if the U.S. saw a Syrian attack as imminent. Nevertheless, the White House showed it wouldn't turn a blind eye. Since Trump's inauguration, U.S. involvement in Syria has deepened. Earlier this month, the U.S. shot down a Syrian fighter jet for the first time. It has twice downed Iranian drones.

The U.S. cruise missile strike in April was the first intentional American assault on Syrian President Bashar Assad's government or military.

The White House issued a brief written statement Monday night saying it had detected potential preparations for another chemical attack and emphasizing the Syrian government would "pay a heavy price" if it proceeded. Hours later, the Pentagon elaborated without offering many specifics.