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.
"However, we continue to closely watch the situation and support the ongoing diplomatic efforts to hold those who use chemical weapons, in Syria and otherwise, accountable," Pentagon spokesman Christopher Sherwood said in a statement.
Syria has repeatedly denied using chemical weapons throughout the conflict that began in 2011, including the most recent suspected chemical attack Saturday in a rebel-held suburb of Damascus that killed at least 40 people.
Late Sunday, the White House said President Donald Trump and French President Emmanuel Macron strongly condemned chemical attacks in Syria and agreed Syrian President Bashar al-Assad's government "must be held accountable for its continued human rights abuses."
"They agreed to exchange information on the nature of the attacks and coordinate a strong, joint response," the White House said about a phone call between the two leaders.
Macron's office added that the two sides "exchanged information and analysis confirming the use of chemical weapons."
Trump used Twitter earlier Sunday to say there would be a "big price to pay" for what he called the "mindless chemical attack" Saturday.
In a rare direct condemnation of Russia's leader, Trump declared, "President Putin, Russia and Iran are responsible" for their support of "Animal Assad."
He further called for Syria to open the area of the alleged chemical attack to allow in verification and medical teams.
The Russian foreign ministry rejected claims of a chemical attack, saying, "The spread of bogus stories about the use of chlorine and other poisonous substances by (Syrian) government forces continues.
"We have warned several times recently against such dangerous provocations," the Moscow statement said. "The aim of such deceitful speculation, lacking any kind of grounding, is to shield terrorists and to attempt to justify possible external uses of force."
Iran said U.S. claims about the attack were aimed at justifying new American military action. A year ago, after an earlier chemical weapons attack by Syria, Trump launched 59 Tomahawk missiles into Syria, targeting the military base that was home to the warplanes that carried out the attack.
Trump did not say how the U.S. might respond to Saturday's suspected chemical attack. But Homeland Security and counterterrorism adviser Thomas Bossert told ABC News, "I wouldn't take anything off the table."
The United Nations Security Council will meet Monday about the alleged attack, after nine countries demanded an urgent session. The European Union said "evidence points toward yet another chemical attack" by the Syrian regime.
Trump also said that if his predecessor, former President Barack Obama, "had crossed his stated Red Line In The Sand," to hold Assad accountable for previous chemical attacks, "the Syrian disaster would have ended long ago! Animal Assad would have been history!"
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...
....to pay. Open area immediately for medical help and verification. Another humanitarian disaster for no reason whatsoever. SICK!
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!
Trump's rebuke of Putin was unusual.