President Donald Trump called Russian-backed Syrian President Bashar al-Assad an "animal" Sunday and said Russian President Vladimir Putin shared the blame for a suspected chemical attack in a Damascus suburb that reportedly killed at least 40.
Trump said Syria had "a big price to pay" but there were no immediate indications of a U.S. military response to the incident Saturday in Douma, east of Damascus.
However, White House Homeland Security adviser Thomas Bossert said no military options were "off the table."
"It's a quite serious problem. We've seen the photos of that attack," Bossert said on ABC News' "This Week" program.
"This is one of those issues on which every nation, all peoples, have all agreed and have agreed since World War II this is an unacceptable practice," Bossert said. "I wouldn't take anything off the table."
Rebel-held Douma has been under siege for weeks by Assad's forces, backed by Russian air power.
Opposition activists, aid groups and independent monitors said that dozens of Douma residents were killed and more than 500 were suffering the aftereffects of barrel-bomb air attacks that allegedly released chlorine and possibly a nerve agent.
A joint statement released Sunday by the Syrian American Medical Society said more than 42 people were found dead in their homes.
In addition, "more than 500 cases -- the majority of whom are women and children --were brought to local medical centers with symptoms indicative of exposure to a chemical agent," the statement said.
In a series of Tweets early Sunday, Trump said "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. President Putin, Russia and Iran are responsible for backing Animal Assad," he said.
"Big price to pay," Trump said. "Open area immediately for medical help and verification. Another humanitarian disaster for no reason whatsoever. SICK!"
Trump also blamed former President Barack Obama for failing to take aggressive action when Assad crossed Obama's "Red Line" on the use of chemical weapons.
In another Tweet, Trump said that "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!"
The latest attack came a year after Trump ordered a Tomahawk cruise missile attack on a Syrian airbase in response to a previous chemical attack in northern Syria. The U.S. warned Russian forces to keep clear of the area before the missiles were launched.
At least 80 reportedly were killed last April in the attack on the town of Khan Sheikhoun, allegedly with the nerve agent sarin.
On the Sunday TV talk shows, several Republican senators said that the U.S. should consider another military response.
"Last time this happened, the president did a targeted attack to take out some of the facilities," Sen. Susan Collins, R-Maine, said on CNN's "State of the Union."
Syria and Russia vehemently denied that a chemical attack took place. The Syrian government-run news agency Sana alleged the reports were a lie made up by Jaish al-Islam, a rebel group that has controlled Douma.
Russia's Foreign Ministry also denied reports of a chemical attack.
"Fake news on the use of chlorine or other chemical agents by the government forces continue," the ministry said in a statement provided to the TASS news agency.
The suspected chemical attack and the potential for a U.S. response added to the political and military realities of Syria's seven-year-old civil war that will make it difficult for Trump to realize his goal of withdrawing the 2,000 U.S. troops in Syria "very soon."
Another argument against a pullout was the potential for clashes in the northeastern crossroads town of Manbij, where U.S. troops were reinforcing positions against a possible attack from elements of the Turkish-backed Free Syrian Army (FSA), which was dug in on the town's outskirts.
The U.S. has maintained a presence in Manbij since it was retaken from the Islamic State of Iraq and Syria (ISIS) in 2016 by the U.S.-backed and mostly Kurdish Syrian Democratic Forces after a lengthy siege.
The U.S. now has two outposts in the Manbij area and Stryker fighting vehicles and MRAPs, Mine Resistant Ambush Protected vehicles, patrol the town flying U.S. flags, according to western news reports and videos.
Special Operations Master Sgt. Jonathan Dunbar and British Sgt. Matt Tonroe were killed on March 29 by a roadside bomb in Manbij, reportedly while on a mission to capture or kill an ISIS operative.
Turkish President Recep Tayyip Erdogan has repeatedly threatened to attack Manbij as part of
"Operation Olive Branch" that began in January as an effort by the Turkish military and their FSA proxies to clear border areas of Kurdish enclaves.
Erdogan maintains that the People's Protection Units, or YPG, the dominant force in the SDF, is a terrorist organization. He also views the YPG as linked to separatists within Turkey.
Last week, following a three-way summit in Ankara hosed by Erdogan with Putin and Iranian President Hassan Rouhani, Erdogan said that "The future of Syria and our region cannot be left to a few terrorists."
Although Trump said last Tuesday that he wanted to get out of Syria, Marine Lt. Gen. Kenneth McKenzie, the Pentagon's Joint Staff Director, said at a Pentagon briefing last Thursday that the military has yet to receive any guidance on a withdrawal.
"The president has actually been very good in not giving us a specific timeline, so that's a tool that we can use to our effect as we move forward," McKenzie said.
"I've heard rumors of people talking about withdrawal," said Dana White, the Pentagon's chief spokesperson.
"I know the president said 'very soon'" because we have been very successful with defeating ISIS. But it's not over, and we are committed to ensuring the defeat of ISIS," she said.
--Richard Sisk can be reached at Richard.Sisk@Military.com.