U.S. President Donald Trump on Monday condemned the "heinous" alleged chemical weapons attack in Syria, saying he would make "major decisions" within 24 to 48 hours about how the U.S would respond.
Trump told his Cabinet at a White House meeting that the U.S. would figure out who was responsible for the attack, whether it was Syria, Russia, Iran or "all of them together."
"We're looking at that very strongly, very seriously," he said.
WATCH: Trump condemns attack on 'innocent Syrians'
Earlier, Defense Secretary Jim Mattis said the United States is not ruling out military action against Syria, after Trump tweeted there would be a "big price to pay" for what he called the "mindless chemical attack" in a rebel-held suburb of Damascus that killed at least 40 people Saturday.
"The first thing we have to look at is why are chemical weapons still being used at all when Russia was the framework guarantor of removing all the chemical weapons," Mattis said, as he met with Qatari Sheikh Tamim bin Hamad al-Thani at the Pentagon.
Britain says Foreign Secretary Boris Johnson spoke to Acting U.S. Secretary of State John Sullivan by phone and the two "agreed that, based on current media reports and reports from those on the ground, this attack bore hallmarks of previous chemical weapons attacks by the Assad regime."
Syria has denied using chemical weapons throughout the conflict that began in 2011, including the most recent suspected chemical attack. Russia said there is no evidence Syria carried out such an attack.
The U.S. president has blamed Syrian President Bashar al-Assad for the attack and in a rare direct condemnation of Russia's leader Vladimir Putin, Trump tweeted Sunday that "President Putin, Russia and Iran are responsible" for their support of "Animal Assad."
The U.N. Security Council met Monday afternoon at the request of multiple members.
U.S. Ambassador Nikki Haley said both Russia and Iran could stop the Syrian government’s “murderous destruction”, adding that Moscow’s hands are “covered in the blood of Syrian children.”
“We have reached the moment when the world must see justice done,” Haley told the council. “History will record this as the moment when the Security Council either discharged its duty or demonstrated its utter and complete failure to protect the people of Syria. Either way, the United States will respond.”
Moscow’s envoy Vassily Nebenzia said the Organization for the Prohibition of Chemical Weapons (OPCW), which is tasked with investigating such allegations, could send a fact-finding mission “tomorrow” to Damascus.
“There the Syrian authorities and Russian troops will provide conditions to travel to the area of the alleged incident,” Nebenzia said.
But Nebenzia went on to say that Russian experts have already visited the site, collected soil samples, interviewed witnesses and medical personnel, and that no chemical weapons attack had taken place.
New Attribution Mechanism
While the OPCW fact-finding mission is mandated to investigate alleged chemical attacks, it does not have the authority to attribute blame. That was the domain of the Joint Investigative Mechanism, which was dissolved after Moscow vetoed a renewal of its mandate in November. The United States put forward a new proposal Monday to Security Council members for a new attribution mechanism, but it was not immediately clear whether it would win the necessary Russian support.
Meanwhile, Syria and Russia say two Israeli war planes operating in Lebanese air space carried out an attack early Monday on an air base in central Syria.
Israel's military did not comment on the strikes against the T4 base in Homs province.
The Britain-based Syrian Observatory for Human Rights said 14 people were killed, including Iranian forces.
In February, Israel accused Iranian forces of using the same site to send a drone to Israeli territory. It responded by attacking Syrian air defense and Iranian military targets within Syria, and Israeli Prime Minister Benjamin Netanyahu pledged to "continue to harm anyone who tries to harm us."
Initial Syrian state media reports Monday blamed the United States, which along with France denied responsibility.
The alleged chemical attack occurred amid new attacks on the last rebel enclave in eastern Ghouta.
First responders said they discovered families suffocated in their homes and shelters with foam on their mouths. Relief workers said more than 500 people, mostly women and children, were brought to medical centers with difficulty breathing, foaming at the mouth and their eyes burning.
The Civil Defense and Syrian American Medical Society said patients gave off a chlorine-like smell, and some had blue skin, an indication of oxygen deprivation.
"Dropping poison gas in a way that attacks women and children down in the shelters is a way to try to panic the civilians into leaving and cut the ground underneath the rebels," University of Pennsylvania political science professor Ian Lustick told VOA.
VOA's Victor Beattie, William Gallo, Jeff Seldin and Margaret Besheer contributed to this report.