The US military said it partnered with Jordan, Bahrain, Saudi Arabia, Qatar and the United Arab Emirates to conduct 14 strikes against Islamic State targets in Syria overnight.
They used "a mix of fighter, bomber, remotely piloted aircraft and Tomahawk Land Attack Missiles," US Central Command said, to damage or destroy the militant group's training compounds, command facilities, armed vehicles and supply trucks.
The group - which has made territorial gains in northern and eastern Syria and neighbouring Iraq - was hit in its north-eastern Syrian stronghold of al-Raqqa.
The Syrian Observatory for Human Rights, a pro-opposition monitoring group, said at least 20 Islamic State fighters were killed.
Damascus said it had been informed about the US-led airstrikes inadvance.
"The American side informed Syria's permanent representative to the United Nations in New York that there would be strikes against the Islamic State in al-Raqqa," Syrian state news agency SANA quoted the Foreign Ministry as saying.
The strikes targeted Islamic State militants also in the town of al-Bu Kamal on the Iraqi border, Kirkuk in northern Iraq, as well as positions near the Kurdish enclave of Kobane in northern Syria, where an offensive launched by the jihadists last week has caused 130,000 civilians to flee across the Turkish border.
Separately, the US military - acting alone - conducted eight airstrikes west of the northern province of Aleppo against al-Qaeda targets, including training camps and a munitions factory.
Syrian sources said that bases of the al-Qaeda affiliate, al-Nusra Front - which fights alongside rebel groups against the government of President Bashar al-Assad - were hit.
US Central Command did not mention al-Nusra by name, but referred to "a network of seasoned al-Qa'ida veterans - sometimes referred to as the Khorasan Group - who have established a safe haven in Syria to develop external attacks, construct and test improvised explosive devices and recruit Westerners to conduct operations.
"The Observatory said at least 30 members of al-Nusra were killed in the attack and they were "mainly of non-Arab nationalities."
US President Barack Obama - who has pledged to "degrade and destroy" the Islamic State group - had earlier this month signalled an expansion of airstrikes to Syria as the militants' territorial expansion has sparked fears of the emergence of a regional militant enclave.
The possibility of expanding airstrikes to Syria had drawn condemnation from Russia and Iran, while Damascus had said it would view any foreign intervention as an act of aggression.
Before the Pentagon announcement, Russian President Vladimir Putin told UN Secretary General Ban Ki-moon that attacks against the Islamic State "should not be launched without the consent of the Syrian government," the Kremlin press service said Tuesday.