BEIRUT -- Missiles struck an air base in central Syria early Monday, but the Pentagon quickly denied claims from Syrian state media that the strikes were "an American aggression." A war monitoring group said Iranian-backed militia members were killed in the strikes, and Russia and then Syria accused Israeli jets of firing the missiles.

Syria's state-run SANA news agency said the missile attack on the T4 military air base in Homs province had resulted in a number of casualties. The London-based Syrian Observatory for Human Rights (SOHR) said 14 people were killed, including foreign nationals -- an allusion to Iranian, or at least Iran-backed, fighters.

Russia's allegation that two Israeli F-15s had fired a total of eight missiles at the T4 base came several hours after Pentagon spokesman Christopher Sherwood said in a statement that the "Department of Defense is not conducting air strikes in Syria."

The missile attack followed a suspected poison gas attack Saturday on the last remaining foothold for the Syrian opposition in the eastern suburbs of Damascus. At least 40 people were killed, including families found in their homes and shelters, opposition activists and local rescuers said.

As CBS News correspondent Holly Williams reports, videos from Douma show lifeless bodies with no visible injuries, all of them apparently inside buildings, survivors struggling to breathe, many of them children. Victims are seen being washed down, apparently to remove a substance from their skin.

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President Trump has promised a "big price to pay" for the suspected chemical attack and called Syrian President Bashar Assad an "animal" in tweets sent earlier Sunday:

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. Big price... — Donald J. Trump (@realDonaldTrump) April 8, 2018

....to pay. Open area immediately for medical help and verification. Another humanitarian disaster for no reason whatsoever. SICK! — Donald J. Trump (@realDonaldTrump) April 8, 2018

CBS News can not independently verify the reports of a chemical attack in Syria, and the Syrian government denied the allegations, calling them fabrications.

Survivors, though, reportedly smelled of chlorine -- a chemical that can be deadly in confined spaces.

Dr. Ahmad Tarakji, of the Syrian American Medical Society, told CBS News via Skype that at his field hospital in eastern Ghouta, they "received many patients who suffered from symptoms compatible with exposure to chlorine gas -- high concentration chlorine gas."

On Monday, Russian Foreign Minister Sergey Lavrov called the accusations against Bashar Assad's regime a "provocation" and, referring to the airstrikes on the base in Homs, he added that the situation in Syria was, "becoming too dangerous" as "actors whom nobody invited" show up on the complex battlefield.

Israel's government did not confirm that it had carried out any strikes in Syria, but it has targeted Assad's forces, and their Iranian allies, inside the country before. Some Israeli media noted

Russia's Ministry of Defense said Monday that "two F-15 aircraft of the Israeli Air Force, without entering Syrian airspace, struck eight controlled missiles at the airfield." It said the missiles were fired from within Lebanese airspace, and that five of them were destroyed by Syria's air defenses before landing.

Three missiles "reached the western part of the airfield," according to the Russian military, which also has personnel in the area in support of Assad's forces. No Russian "advisers" were hit in the early morning missile strike, according to Russia.

In the statement released early Monday morning, U.S. military spokesman Sherwood said the Pentagon would "continue to closely watch the situation and support the ongoing diplomatic efforts to hold those who use chemical weapons, in Syria and otherwise, accountable."

The U.S. launched several dozen Tomahawk cruise missiles at a Syrian air base in 2017 after a chemical attack in the northern Syrian town of Khan Sheikhoun killed dozens of people. Israel has also struck inside Syria in recent years.

The suspected poison gas attack Saturday on the besieged town of Douma came almost exactly a year after the U.S. missile attack prompted by the Khan Sheikhoun deaths.

First responders said they found families suffocated in their homes and shelters, with foam on their mouths. The opposition-linked Syrian Civil Defense were able to document 42 fatalities but were impeded from searching further by strong odors that gave their rescuers difficulties breathing, said Siraj Mahmoud, a spokesman for the group, which is known as the White Helmets.

"Open area immediately for medical help and verification. Another humanitarian disaster for no reason whatsoever. SICK!" the president wrote.

Mr. Trump later blamed his predecessor President Barack Obama for not taking action against the Assad regime earlier in the civil war.

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! — Donald J. Trump (@realDonaldTrump) April 8, 2018

Hours after the attack, the Army of Islam rebel group agreed to surrender the town and evacuate their fighters to rebel-held northern Syria, Syrian state media reported. The group also agreed to give up its prisoners, a key demand of the government.

The government agreed to halt its assault after three days of indiscriminate air and ground attacks.

"There's nothing left for civilians and fighters. We don't have anything to stand fast," said Haitham Bakkar, an opposition activist inside the town. He spoke to the Associated Press via WhatsApp.

"People now are going out in the streets looking for their loved ones in the rubble," Bakkar said. "And we don't have any space left to bury them."

More than 100 buses entered the town Sunday night to transport fighters and their families to Jarablus, a town under the shared control of rebels and Turkey, said Syrian state-affiliated al-Ikhbariya TV.

The preparations follow a pattern of evacuations around the capital and other major Syrian cities as the government reasserts its control after seven years of war.

Human rights groups and United Nations officials say the tactic amounts to forced displacement, a war crime. The U.N. Security Council planned to hold an emergency meeting Monday to discuss the attack.

It's unclear what the administration's next steps are with regard to responding to the attack. In response to a similar chemical attack in April of last year, Mr. Trump ordered a missile strike on a Syrian military target in Shayrat, about 50 miles due south of the village that was hit in a gas attack.

Treasury Secretary Steven Mnuchin said Sunday on CBS News' "Face the Nation" that he and the administration "will be reviewing with the president all different alternatives" for a response.