(CNN) – President Donald Trump called out Russian President Vladimir Putin by name on Sunday for backing Syrian President Bashar al-Assad and warned of a “big price” after reports of a chemical weapons attack in Syria almost a year to the day since the US struck a Syrian air field after a previous attack.

“Many dead, including women and children, in mindless CHEMICAL attack in Syria,” Trump tweeted. “Area of atrocity is in lock down and encircled by Syrian Army, making it completely inaccessible to outside world. President Putin, Russia and Iran are responsible for backing Animal Assad. Big price…”

He continued, “….to pay. Open area immediately for medical help and verification. Another humanitarian disaster for no reason whatsoever. SICK!”

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Trump’s tweet about Putin apparently marks the first time the President has publicly attacked the Russian leader by name. Trump also pinned blame for the situation on former US President Barack Obama’s Syria policies.

“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 tweeted.

Vice President Mike Pence said in a tweet later Sunday morning that he and Trump were “monitoring the likely chemical attack” and echoed Trump’s language about potential consequences.

“.@POTUS & I closely monitoring likely chemical attack in Syria. We condemn in the strongest possible terms the assault on innocent lives, including children. The Assad regime & its backers MUST END their barbaric behavior. As POTUS said, big price to pay for those responsible!” Pence tweeted.

Syrian activist groups on Saturday said toxic gas inside barrel bombs dropped from helicopters over a rebel-held city in Syria killed dozens of civilians and wounded scores more. Syrian state news said an “official source” denied the allegations.

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In the early hours of Monday morning, Syria State TV reported that “several missiles” had been fired at the T4 air base in Homs, in the country’s west, adding that it was “likely an attack by the US.” The Pentagon issued a statement Sunday denying it was conducting air strikes in the region.

National Security Council principals will hold a “small group” meeting to discuss Syria on Monday afternoon, led by John Bolton on his first day as White House national security adviser, two administration officials told CNN.

A small group meeting is typically held to discuss options for the President, one of the officials said.

Trump also spoke with Iraqi Prime Minister Haider Al-Abadi, according to a White House readout. The pair discussed the “situation in Syria and the alarming reports of possible chemical attacks near Damascus,” as well as the countries’ continued fight against ISIS, the White House said.

The United Nations Security Council will hold an emergency meeting Monday on the alleged chemical weapons use, the US mission to the United Nations announced. Russia has requested a separate Security Council meeting later in the day, two UN diplomats told CNN.

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In April of last year, the US launched tomahawk missiles at a Syrian air base as the US and others accused Assad’s forces of perpetrating a deadly chemical weapons attack, which the Syrian military denied.

Speaking on ABC’s “This Week” in an interview taped prior to the President’s tweets, White House homeland security adviser Tom Bossert said he was struck by the “timing” of the reported attack, around the anniversary of last year’s missile strike. He said the President and his advisers had been discussing the strike and would not rule out another one.

“I wouldn’t take anything off the table,” he said.

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