President Trump on Sunday vowed “Animal Assad” would pay a “big price” for an apparent chemical weapons attack that killed dozens in Syria.

And in a rare public rebuke, Trump also called out Russian leader Vladimir Putin for propping up the despot.

Rescue workers said at least 42 people — mostly women and children — died in the gas attack on a rebel-held area of Douma, a suburb of the capital, Damascus, and reported finding entire families suffocated in their homes with white foam covering their mouths.

They warned the death toll could rise because the strong smell of chlorine gas has hampered efforts to look for other victims.

Trump called for Syrian strongman Bashar al-Assad to allow aid groups into the area and blamed Putin by name and Iran for supporting the government.

​”​Many dead, including women and children, in mindless CHEMICAL attack in Syria. Area of atrocity is in lockdown and encircled by Syrian Army, making it completely inaccessible to outside world​,” Trump posted on Twitter.

“President Putin, Russia and Iran are responsible for backing Animal Assad. Big price​ to pay. Open area immediately for medical help and verification. Another humanitarian disaster for no reason whatsoever. SICK!​”​

Sen. Lindsey Graham (R-S.C.), said Trump’s next move could be a make-or-break moment because adversaries Russia, Iran and North Korea are watching to see how he responds.

“It’s a defining moment in his presidency, because he has challenged Assad in the past not to use chemical weapons,” Graham said on ABC’s “This Week,” adding that Syria sees “our resolve breaking.”

Trump stunned many on his national security staff Thursday when he suddenly announced that he intends to pull US troops out of Syria “very soon.”

Sen. John McCain (R-Ariz.) said Syria was emboldened by Trump’s talk of a troop withdrawal and urged the president to respond forcefully to “demonstrate that Assad will pay a price for his war crimes.”

Last April, Trump ordered a cruise missile attack on a Syrian airfield in retaliation for Assad’s use of Sarin gas against civilians.

The president will meet with his top military advisers Monday, the first day on the job for his incoming national security adviser, John Bolton.

The United Nations Security Council will also discuss Syria on Monday, as international outrage is growing.

In his tweets, Trump also took a shot at his predecessor for not getting rid of Assad.

“If President Obama had crossed his stated Red Line In The Sand, the Syrian disaster would have ended long ago! Animal Assad would have been history!​,” Trump said. ​

Syria denied launching an attack and Russia called the reports “bogus.”