WASHINGTON (AP) — Combined U.S.-Arab airstrikes on the Islamic State group's military strongholds in Syria achieved their aim of showing the extremists that their savage attacks will not go unanswered, the top American military officer said Tuesday. Separately, the U.S. launched strikes against a group said to be plotting to attack the U.S. and Western interests.

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.

American warplanes also carried out eight airstrikes to disrupt what the military described as "imminent attack plotting against the United States and Western interests" by a network of al-Qaida veterans "with significant explosives skills," said Army Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff.

The White House said President Barack Obama would speak about the airstrikes before flying to New York on Tuesday morning 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. Dempsey said their role was indispensable to the U.S. goal of showing that the battle to degrade and defeat the Islamic State group is not just a U.S. fight.

Dempsey called the 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. "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 al-Qaida veterans — sometimes known as the Khorasan Group — who have established a haven in Syria. It provided no details on the plotting.

Dempsey said the decision to launch both operations simultaneously was influenced by a concern that word of strikes in eastern Syria could prompt the al-Qaida veterans to disperse. The Khorasan Group "may have scattered" if the attack missions had been done sequentially rather than simultaneously, he said.

Central Command said the bombing mission against that group 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. EDT. 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 Tuesday 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.

Russia's foreign ministry warned Tuesday that what it called "unilateral" air strikes would destabilize the region. "The fight against terrorists in the Middle East and northern Africa requires coordinated efforts of the entire global community under the auspices of the U.N.," the foreign ministry said in a statement.

Activists said the airstrikes hit targets in and around the Syrian city of Raqqa and the province with the same name. Raqqa is the Islamic State group's self-declared capital in Syria.

Rami Abdurrahman, who heads the Britain-based Syrian Observatory for Human Rights, told The Associated Press, "There is confirmed information that there are casualties among Islamic State group members."

Dempsey said Arab participation needs to extend beyond direct military roles to assisting in an international effort to undercut finances, recruiting and ideological support for the Islamic State group.

"What we're talking about now is the beginning of an air campaign," he said, adding that it must lead to what he called "the other air campaign" — an effort to fill public airwaves across the Muslim world with arguments for why the extremists must be defeated.

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. The Obama administration, which at a NATO meeting in Wales earlier this month also got commitments from European allies as well as Canada and Australia, has insisted that the fight against the Islamic State militants could not be the United States' fight alone.

In a speech Sept. 10, Obama vowed to go after the Islamic State militants wherever they may be. His military and defense leaders told Congress last week that airstrikes within Syria are meant to disrupt the group's momentum and provide time for the U.S. and allies to train and equip moderate Syrian rebels.

The U.S. military has been launching targeted airstrikes in Iraq since August, focusing specifically on attacks to protect U.S. interests and personnel, assist Iraqi refugees and secure critical infrastructure. Last week, as part of the newly expanded campaign, the U.S. began going after militant targets across Iraq, including enemy fighters, outposts, equipment and weapons.

To date, U.S. fighter aircraft, bombers and drones have launched about 190 airstrikes within Iraq.

Urged on by the White House and U.S. defense and military officials, Congress passed legislation late last week authorizing the military to arm and train moderate Syrian rebels. Obama signed the bill into law Friday, providing $500 million for the U.S. to train about 5,000 rebels over the next year.

The militant group, meanwhile, has threatened retribution. Its spokesman, Abu Mohammed al-Adnani, said in a 42-minute audio statement released Sunday that the fighters were ready to battle the U.S.-led military coalition and called for attacks at home and abroad.

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Burns reported from aboard a U.S. military aircraft. Associated Press writers Zeina Karam, Julie Pace and Matthew Lee in New York, Josh Lederman in Washington and Bassem Mroue in Beirut contributed to this report.