IRBIL, Iraq — U.S.-led airstrikes in Syria are likely to last "for years," a senior Pentagon official said Tuesday, as the United States began to assess the impact of three waves of aerial assaults launched in the early morning hours that targeted both Islamic State installations in eastern Syria and facilities housing a shadowy al-Qaida group further west.

Outside observers said the attacks, which Pentagon officials said struck 22 targets overall, killed at least 120 jihadi fighters, including at least 70 who were associated with the Islamic State and another 50 who belonged to the al-Qaida-affiliated Nusra Front and who were apparently killed in U.S. missile attacks aimed at Nusra's Khorasan unit, which U.S. officials said was plotting an imminent attack on the West.

But Syrian rebels and media activists said the airstrikes also killed at least 10 civilians when a missile struck a building temporarily housing displaced people in Idlib province. They said that the Islamic State had evacuated many of the buildings and encampments struck in Raqqa, the Islamic State's de facto capital.

Activists in another town hit in the U.S.-led offensive, Ash Shadadi, said all of the Islamic State's bases were empty when bombed.

They blasted the United States for including Nusra on its target list. "Nusra is still popular in Syria,'" said Col. Hassan Hassan Hamadi, a defected Syrian army officer whose Legion 5 force has about 6,400 fighters.

Pentagon officials said they had no information to confirm the claim of civilian casualties but would investigate.

Instead, Pentagon officials displayed photographs of buildings that they said showed the pinpoint nature of the attacks. In one set, a slide showed what officials said was a communications array atop what they identified as the Islamic State's financial center in Raqqa, the organization's de facto capital. A second image showed the array destroyed by Tomahawk missiles but with little apparent damage to the building itself.

A resident of Raqqa, reached via Skype, said that damage in the city appeared to be confined largely to Islamic State facilities, and that there was no extensive damage to the city itself.

"It's actually not that bad," said the resident, who asked not to be identified for security reasons. "The strikes were very loud and surprising but did not seem to hit many civilian homes."

He said Islamic State fighters had been deployed to prevent onlookers from assessing the damage, however.

"It's very tense right now," the resident said.

It was uncertain how soon the U.S.-led coalition might strike again. In Iraq, U.S. aircraft have undertaken more than 190 attacks in coordination with Kurdish or Iraqi ground forces since President Barack Obama approved U.S. military action there Aug. 7. But whether the same pace would apply to Syria, where the United States still is uncertain who its ground force partner will be, was not clear. U.S. officials have worried that airstrikes against the Islamic State inside Syria might unintentionally empower entities it did not want to strengthen, including al-Qaida's Nusra and the government of Syrian President Bashar Assad.

"You are seeing the beginning of a sustained campaign, and strikes like this in the future can be expected," said Army Lt. Gen. William C. Mayville Jr., the director of operations for the Joint Chiefs of Staff. "The operational pace, the tempo of this thing, will be dictated by the facts on the ground and what the targets on the ground mean."

Asked how long the effort to "degrade and eventually destroy" the Islamic State could take, Mayville replied, "I would think of it in terms of years."

An initial assessment indicated that the strikes "were very successful," Navy Rear Adm. John Kirby, the chief Pentagon spokesman, told a briefing.

In describing the Syrian strikes, which Obama had warned were coming in a nationally televised address Sept. 10, Pentagon officials were careful to divide them into two distinct groups: those that targeted the Islamic State, the militant group that now controls a third of Iraq and an equal portion of Syria; and those aimed at Khorasan, a unit attached to Syria's Nusra Front rebels and that U.S. officials said included senior al-Qaida figures.

The first group of attacks targeted 14 specific locations in Syria, U.S. officials said, and was the work of a coalition of countries that included the United States and five Arab nations: Saudi Arabia, Qatar, Jordan, the United Arab Emirates and Bahrain.

Only American assets were used in the second set of attacks, which targeted eight locations associated with Khorasan, a group that includes Muhslin al Fahdli, a 33-year-old former close associate of al-Qaida founder Osama bin Laden who was sent to Syria, U.S. officials said, to plan attacks on the United States and Europe. The U.S. government in October 2012 offered a $7 million reward for information leading to Fahdli's death or capture.

Khorasan was under the protection of Nusra, al-Qaida's official affiliate in Syria, and one expert was quick to point out that there was little difference between the two. He said U.S. officials' use of the name Khorasan, instead of Nusra, was possibly an effort to avoid negative reaction from other rebel groups in Syria that frequently work with Nusra.

"It's not a separate group from Nusra," said Aymenn al Tamimi, who studies jihadist groups for the Middle East Forum, a Philadelphia-based think tank. "The U.S. attempt to distinguish it is just a pretense for striking Nusra because they know Nusra has support on the ground and works with a wide range of rebel groups, some of whom the U.S. might want to consider aiding against (the Islamic State)."

Indeed, activists reporting on Tuesday's attacks described the targets as Nusra military bases, and the Syrian Observatory for Human Rights described the 50 killed in the attacks as Nusra fighters. U.S. Central Command said the eight targets were in western Aleppo province.

The toll was heavier for Islamic State militants, according to the observatory, which said more than 70 Islamic State members were killed and 300 others were wounded. The observatory, citing "reliable sources," said at least 100 of the wounded were in critical condition and had been taken to Iraq for treatment, and that the number of dead was sure to rise substantially.

The Syrian government, which is battling both the Islamic State and Nusra, offered no public criticism of the raids. On Tuesday, the United States confirmed that it had informed the Syrian government of the strikes through Syria's ambassador to the United Nations, but it said it had not coordinated the raids with the Syrian government and "did not provide advance notification to the Syrians at a military level."

"We warned Syria not to engage U.S. aircraft. We did not request the regime's permission," a State Department statement said.

In all, 47 Tomahawk cruise missiles were launched from two ships, the destroyer USS Arleigh Burke and the guided missile cruiser USS Philippine Sea, which were in the Red Sea and the North Arabian Gulf, Centcom said. Aircraft came from the Air Force, the Navy and the Marine Corps.

The targets, Central Command said, "included ISIL fighters, training compounds, headquarters and command-and-control facilities, storage facilities, a finance center, supply trucks and armed vehicles." The strikes didn't target specific Islamic State leaders, although some focused on "command-and-control nodes" identified by observing "patterns of life," Mayville said.

Mayville and Kirby also provided new details of the three waves of missile and air strikes. Four of the Arab allies — Saudi Arabia, Bahrain, Jordan and the United Arab Emirates — contributed planes to the assault. Qatar provided unspecified support to the mission, they said.

The operations involved the first-ever use in combat of the F-22, a $150 million stealth fighter known as the Raptor.

The first wave of strikes began about 2:30 a.m. in Syria, or 8:30 p.m. EDT, and targeted the Syrian cities of Aleppo and Raqqa with cruise missiles, Mayville said.

Most of the Tomahawks were aimed at training camps, workshops and other facilities belonging to the Khorasan group, Mayville said.

"Intelligence reports indicated that the Khorasan group was in the final stages of plans to attack Western targets and potentially against the U.S. homeland," he said. "We're still assessing the effects of our strikes, but we've been watching this group closely for some time. The Khorasan group is clearly not focused on the Assad regime or the Syrian people."

The second and third wave of strikes targeted Islamic State command, control and communications facilities, barracks, training grounds and vehicles. The "preponderance" of the Arab aircraft participated in the third wave, Kirby said, declining to provide further details.

Aleppo, Syria's largest city and once its commercial capital, has been devastated by battles between Assad's forces and insurgent groups, which also are fighting on a second front against the Islamic State. Raqqa, the chief city of a northeastern province of the same name controlled by the Islamic State, is where the Islamist group has made its unofficial capital.

Twitter accounts associated with the Islamic State immediately warned America and other members of the coalition that there would be revenge attacks for the strikes, which came one day after the group's spokesman called upon Muslims to attack any citizens of coalition countries.