International condemnation grew over a suspected poison gas attack in a rebel-held town near Damascus said to have been carried out by the Syrian government, while Syria and its main ally, Russia, blamed Israel for airstrikes on a Syrian air base that reportedly killed 14 people, including four Iranians.

The timing of the airstrikes in central Homs province, hours after President Donald Trump said there would be "a big price to pay" for the chemical weapons attack, raised questions about whether Israel was acting alone or as a proxy for the United States.

Israel is yet to comment on the missile strike which hit Syria's Tayfour airbase also known as T4. The Jewish State typically does not comment on its airstrikes in Syria, which have been numerous in Syria's civil war.

The fast-paced developments threatened to further hike tensions between the U.S. and Russia, which has in the past warned against any U.S. military action against President Bashar Assad's government.

Iran, a key ally of Assad, condemned the airstrikes, which it said killed four Iranians, including a colonel and a member of the Revolutionary Guard's aerospace force.

Opposition activists said 40 people died in Saturday night's chemical attack in the town of Douma, the last remaining rebel bastion in the eastern suburbs of Damascus, blaming Assad's forces. The attack killed entire families in their homes and underground shelters, opposition activists and local rescuers said.

The Syrian government strongly denied it carried out a chemical weapons attack and the Organization for the Prohibition of Chemical Weapons said it has opened an investigation. In a statement, it said a fact-finding mission was gathering information from all available sources to establish whether chemical weapons were used.

Russia's UN ambassador, Vassily Nebenzia, urged inspectors from the watchdog agency to fly to Syria's capital today and visit the site in a nearby rebel-held town.He denied any attack occurred, telling the UN Security Council that experts from Russia's military radiological, biological and chemical unit went to the site and found no chemical substances on the ground, no dead, and no poisoned people in hospitals.

Trump today condemned the "heinous attack" in Syria and said later at a Cabinet meeting that he would "forcefully" respond.

"Nothing is off the table," Trump warned. He said that after conferring with his military advisers, he would soon decide on how to respond, and against whom. "If it's Russia, if it's Syria, if it's Iran, if it's all of them together, we'll figure it out," Trump said. Referring to Russian President Vladimir Putin, he added: "Everybody's going to pay a price — he will, everybody will."

British Prime Minister Theresa May said Assad's government and its backers, including Russia, "must be held to account" if it is found to have been responsible for the suspected poison gas attack. "Yes, this is about the actions, the brutal actions by Assad and his regime. But it also is about the backers of the regime, and of course Russia is one of those backers. ... And they need to look very carefully at the position they have taken," she said. The European Union also laid the blame squarely on Assad's government.

The UN Security Council was holding an emergency meeting today to discuss the chemical attack. It was the second such airstrike this year on the Syrian air base, known as T4, where Iranian fighters are believed to be stationed. Israel hit the base in February, after it said an Iranian drone that violated Israeli airspace took off from it.

Russia's Defense Ministry said two Israeli aircraft targeted the base yesterday, firing eight missiles. It said Syria shot down five of them while the other three landed in the western part of the base.

Syrian state TV quoted an unnamed military official as saying that Israeli F-15 warplanes fired several missiles at T4. It gave no further details.

Israel's Foreign Ministry had no comment when asked about reports of the airstrikes. Amos Yadlin, a former Israeli fighter pilot and ex-head of Israeli military intelligence, stopped short of saying Israel was responsible for the airstrike. But he suggested that the chemical attack had crossed a red line and prompted Israel to take action to send a message to Syria and arch-enemy Iran. "The Iranians are determined to base themselves in Syria," he told the Army Radio station. "Israel is determined not to let them do that. And there is a strategic collision that perhaps tonight may have come together because of the chemical issue."

Since 2012, Israel has struck inside Syria more than 100 times, mostly targeting suspected weapons convoys destined for the Lebanese militant group Hezbollah, which has been fighting alongside Syrian government forces. The base, which was used as a launching pad for attacks against Islamic State militants who were at one point stationed nearby, is near the Shayrat air base, which was targeted by U.S. missiles last year in response to a chemical weapons attack.

Syria's state news agency SANA initially said the attack on the T4 air base was likely "an American aggression," but the Pentagon denied involvement, and the agency then dropped the accusation, blaming Israel instead. SANA said the missile attack resulted in a number of casualties, but provided no specific figures.