International condemnation has grown over a suspected poison gas attack in a rebel-held town near Damascus said to have been carried out by the Syrian government, reportedly killing 40 people.
The accusations came as Syria and its main ally, Russia, blamed Israel for airstrikes on a Syrian air base on Monday 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.
The fast-paced developments threatened to further hike tensions between the US and Russia, which has in the past warned against any US military action against President Bashar Assad's government.
Iran, a key ally of Mr 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 Mr Assad's forces.
The Syrian government strongly denied it carried out a chemical weapons attack and the Organisation for the Prohibition of Chemical Weapons said it has opened an investigation.
Mr Trump condemned the "heinous attack" in Syria and said he would make a decision on a US response "probably by the end of today" after huddling with military advisers. "Nothing is off the table," he warned.
"If it's Russia, if it's Syria, if it's Iran, if it's all of them together, we'll figure it out," he 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 Mr 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.
The European Union also laid the blame squarely on Mr Assad's government while the UN Security Council planned to hold an emergency meeting on Monday to discuss the chemical attack.
The airstrike was the second such attack 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 Defence Ministry said two Israeli aircraft targeted the base on Monday, firing eight missiles.
It said Syria shot down five of them while the other three landed in the western part of the base.
Israel's Foreign Ministry had no comment when asked about reports of the airstrikes.
The Britain-based Syrian Observatory for Human Rights, which monitors the war through a network of activists on the ground, said 14 died, including Iranians and three Syrian officers.
Mr Putin's spokesman Dmitry Peskov told reporters that Israel had not notified Russia of the airstrike, even though there may have been Russian military advisers at the base, which he described as "a cause for concern for us".
The Syrian government has denied the chemical weapons allegations, calling them fabrications. The Russian military said its officers visited the hospital in Douma and talked to the staff, and said they did not confirm reports of the assault.