WASHINGTON - The United States, along with its Arab allies, opened a significant new front in its military campaign in the Middle East with a wave of air and missile strikes aimed at Islamic State militants in Syria.

Among the targets of US strikes across Syria early Tuesday was a collection of buildings to the west of Aleppo, some distance from ISIS strongholds.

While the United States worked with Arab partners to attack ISIS targets, the military alone took aim at the Khorasan Group, an organisation formed by senior Al-Qaeda members based in Pakistan who travelled to Syria, CNN reported.

The sites the United States struck overnight included “training camps, an explosives and munitions production facility, a communication building and command and control facilities,” the military said in a statement.

The group was actively plotting against a US homeland target and Western targets, a senior US official told CNN on Tuesday. The United States hoped to surprise the group by mixing strikes against it with strikes against ISIS targets.

The official said the group posed an “imminent” threat. But another US official later said the threat was not imminent in the sense that there were no known targets or attacks expected in the next few weeks.

The plots were believed to be in an advanced stage, the second US official said. There were indications that the militants had obtained materials and were working on new improvised explosive devices that would be hard to detect, including common hand-held electronic devices and airplane carry-on items such as toiletries.

Khorasan’s existence was publicly acknowledged only last week, when US Director of National Intelligence James Clapper said it was operating in Iraq and Syria, with a focus on exporting terror to the West.

A Syrian analyst interviewed on tightly-controlled state TV said the air strikes did not amount to an act of aggression because the government had been notified in advance.

“This does not mean we are part of the joint operations room, and we are not part of the alliance. But there is a common enemy,” said the analyst, Ali al-Ahmad.

Meanwhile, the Syrian Observatory for Human Rights said US air strikes killed at least 120 jihadists in Syria.

The dead included more than 70 members of the Islamic State (IS) group in the north and east of Syria, as well as 50 Al-Qaeda militants. Eight civilians, among them three children, were also killed in the strikes, the Observatory said.

The group said at least 300 people were injured in the strikes, about 100 of whom were in serious condition.

In a related development, President Barack Obama hailed the American-led coalition that conducted airstrikes against the Islamic State, declaring, “We’re going to do what is necessary to take the fight to this terrorist group.”

Obama, who spoke in Washington on Tuesday before leaving for a United Nations conference on Climate Change in New York, said the joint fight against the Islamic State will take time but is vital to the security of the US, the Mideast and the world and that the US is “proud to stand shoulder-to-shoulder” with Bahrain, Qatar, Saudi Arabia, Jordan and the United Arab Emirates in conducting the strikes against Islamic State targets.

“America is proud to stand shoulder to shoulder with these nations on behalf of our common security,” Obama said. “The strength of this coalition makes clear to the world that this is not just America’s fight alone.”

The US announced the strikes hours before Obama was due to arrive in New York for three days of talks with foreign leaders at the annual United Nations General Assembly ministerial meeting. Diplomats said the cooperation by Arab partners provided a significant boost to Obama’s efforts to build an international coalition to take on the Islamic State militants who have moved freely across the border between Iraq and Syria.

Obama significantly ramped up US military involvement in Syria, a country that has been mired in a three-year civil war. He has insisted the US would not be alone in trying to root out the Islamic State, but the public commitments from allies had been few and far between. Before the strikes, only France had committed to air strikes in Iraq, and Saudi Arabia had volunteered to host US-led training missions for Syrian rebels.

The president said American planes had also struck targets of another militant group, Khorasan, declaring that there would be “no safe haven” for the Al-Qaeda-linked group, which officials say has been plotting attacks against Americans.

The expansion of military action to Syria, as leaders of 180 countries are gathering at the United Nations, is likely to galvanise a meeting that was already going to be dominated by Obama’s efforts to build a coalition for the fight against the Islamic State.

Obama said he would meet with leaders from several countries in an effort to “cut off ISIL’s financing, to counter its hateful ideology, and to stop the flow of fighters into the region.”

The attacks were said to have scattered the IS forces and damaged the network of facilities they have built in Syria that helped fuel the group’s seizure of a large part of Iraq this year.

Separate from the attacks on the Islamic State, the United States Central Command, or Centcom, said that American forces acting alone “took action” against “a network of seasoned Al-Qaeda veterans” from the Khorasan group in Syria to disrupt “imminent attack planning against the United States and Western interests.”

Officials did not reveal where or when such attacks might take place.

Al-Qaeda cut ties with the Islamic State earlier this year because the group’s leader, Abu Bakr al-Baghdadi, disobeyed orders from Al-Qaeda to fight only in Iraq. Just days ago, American officials said the Khorasan group, led by a shadowy figure who was once in Osama bin Laden’s inner circle, had emerged in the past year as the Syria-based cell most intent on launching a terrorist attack on the United States or on its installations overseas.

The latest campaign opened with multiple strikes before dawn that focused on the Islamic State’s de facto capital, the city of Raqqa, and on its bases in the surrounding countryside. Other strikes hit in the provinces of Deir al-Zour and Hasaka, whose oil wells the Islamic State have exploited to finance its operations.

The extent of the damage caused by the strikes remained unclear. Centcom said the wave of fighter planes, bombers, drones and cruise missiles struck 14 targets linked to the Islamic State.