At least 58 people were killed Tuesday, including 50 jihadis, as the US-led alliance launched airstrikes on Islamic extremists in Syria, the director of the Syrian Observatory for Human Rights (SOHR), Rami Abdul Rahman, told Efe news agency.
The Syrian government was informed in advance about the operation on its territory.
The fatalities included 50 members of the Nusra Front, affiliated to the Al Qaeda in Syria, who died in a rocket attack on their base in the western part of the northern city of Aleppo.
The rest were civilians, among them a woman and two minors, who were killed in a bombing on the Kafr Diryan village bordering the Syrian provinces of Idlib and Aleppo.
The US late Monday announced the beginning of the international operation against the Islamic State (IS) Sunni radical group on Syrian territory, without mentioning the Nusra Front.
The alliance's warplanes hit bases of the IS in Raqaa, Deir al-Zur, al-Hasakah and the Aleppo governorates, Abdul Rahman said by phone.
He added that positions of the Nusra Front were also targeted.
It is still unknown whether any IS fighters were killed or wounded in the bombings.
According to SOHR, the alliance launched more than 50 attacks over several hours on IS bases, many of which were evacuated last week.
US Defence Department spokesman John Kirby had announced hours earlier that his country and its allies had begun the air attacks.
The Washington Post and New York Times newspapers quoted official sources as saying that five Arab countries had taken part in the attacks -- Bahrain, Jordan, Saudi Arabia, the United Arab Emirates and Qatar.
Damascus announced Tuesday that it supported and was ready for any international effort against the jihadi groups as long as it respected national sovereignty and is consistent with international decisions.
According to a Xinhua report, Washington had informed Syria of airstrikes against positions of the IS in Syria before Tuesday's strikes started.
The Syrian foreign ministry said in a statement Tuesday that Syria's envoy to the UN was informed Monday about the commencement of the US-led airstrikes against IS positions in Syria's northern province of al-Raqqa, adding the airstrikes started before daybreak Tuesday.
The ministry did not mention the Syrian government's response to the airstrikes.
Meanwhile, Jordan announced Tuesday that its warplanes took part in the US-led air operation against the IS jihadis in northern Syria, an Efe report from Amman said.
In a statement reported by the official news agency Petra, a source at army headquarters said that the Jordanian air force destroyed "several targets".
Without mentioning Syria, the statement said that the strikes were aimed at "positions of terrorist groups that are used as platforms to launch operations against Jordanian territory".
"The armed forces will not hesitate to give a decisive answer to these groups, as they will not tolerate harming border security and Jordanian citizens," the statement said.
All the aircraft and their crew returned safely to their bases, the statement added.
