WASHINGTON — Combined U.S.-Arab airstrikes at the heart of the Islamic State group’s military strongholds in Syria achieved their strategic aim of showing the extremists that their savage attacks will not go unanswered, the top American military officer said today.

The U.S. and five Arab nations attacked the Islamic State group’s headquarters in eastern Syria in nighttime raids Monday using land- and sea-based U.S. aircraft as well as Tomahawk cruise missiles launched from two Navy ships in the Red Sea and the northern Persian Gulf.

Separately, American warplanes also launched eight airstrikes “to disrupt the imminent attack plotting against the United States and Western interests” by a network of “seasoned al-Qaida veterans,” the U.S. military said.

The White House said President Barack Obama spoke about the airstrikes before flying to New York today for the United Nations General Assembly meeting.

U.S. officials said five Arab nations either participated in the airstrikes or provided unspecified support. They were Saudi Arabia, Qatar, Bahrain, Jordan and the United Arab Emirates.

Army Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, called strikes an unprecedented coalition with Arab states and said the partnering has set the stage for a broader international campaign against the extremists.

“We wanted to make sure that ISIL knew they have no safe haven, and we certainly achieved that,” Dempsey told reporters as he flew to Washington after a weeklong trip to Europe. ISIL is an alternate acronym for the Islamic State group whose fighters swept across much of Iraq this summer.

Dempsey said the five Arab nations’ agreement to join in the airstrikes came together quickly; as recently as Sunday he told reporters that more Arab participation was needed before Obama would sign off on the strategic air campaign.

“Once we had one of them on board, the others followed quickly thereafter,” he said, adding that the partnership came together over the past three days. “We now have a kind of credible campaign against ISIL that includes a coalition of partners.”

Several hours after the Pentagon announced the airstrikes against Islamic State targets, U.S. Central Command said American warplanes also launched eight airstrikes “to disrupt the imminent attack plotting against the United States and Western interests” by a network of “seasoned al-Qaida veterans” — sometimes known as the Khorasan Group — who have established a haven in Syria. It provided no details on the plotting.

Central Command said the separate bombing mission was undertaken solely by U.S. aircraft and took place west of the Syrian city of Aleppo. It said targets included training camps, an explosives and munitions production facility, a communication building and command and control facilities.

The airstrikes against Islamic State targets were carried out in the city of Raqqa and other areas in eastern Syria. The strikes were part of the expanded military campaign that Obama authorized nearly two weeks ago in order to disrupt and destroy the Islamic State militants, who have slaughtered thousands of people, beheaded Westerners — including two American journalists — and captured large swaths of Syria and northern and western Iraq.

The airstrikes began around 8:30 p.m. Central Command said the U.S. fired 47 Tomahawk cruise missiles from aboard the USS Arleigh Burke and USS Philippine Sea, operating from international waters in the Red Sea and the northern Persian Gulf. U.S. Air Force, Navy and Marine Corps fighter jets, drones and bombers also participated.

Syria’s Foreign Ministry said the U.S. informed Syria’s envoy to the U.N. that “strikes will be launched against the terrorist Daesh group in Raqqa.” The statement used an Arabic name to refer to the Islamic State group.

Rear Admiral John Kirby, the Pentagon press secretary, said today the strikes weren’t coordinated with the regime of President Bashar Assad, but added: “There was no resistance, no interaction with Syrian air forces or military defenses” during the operation.

At a conference on Sept. 11 with Secretary of State John Kerry, key Arab allies promised they would “do their share” to fight the Islamic State militants.