BEIRUT (AP) — Islamic State militants seized a string of villages from rival insurgents north of the Syrian city of Aleppo on Friday in a surprise attack that came despite intensive Russian airstrikes that Moscow insists are targeting the extremist group, activists said.

Iranian state media reported that a senior commander in Iran's powerful Revolutionary Guard was killed by the Islamic State group on the outskirts of Aleppo city, but it was not immediately clear whether Gen. Hossein Hamedani's death was related to the new IS offensive.

An Iranian state television report said he was killed while "carrying out an advisory mission," and the official IRNA news agency read a statement by the Guard in which it blamed IS for his death. Neither report provided further details.

The IS advance — the most significant in months — came amid a wave of Russian airstrikes that have targeted insurgents fighting to topple President Bashar Assad, and a ground offensive by the Syrian army in the country's central region.

Moscow says it is targeting mainly Islamic State militants, but U.S. officials and Syrian rebels have said the strikes have hit mainstream rebels for the most part and are aimed at shoring up Assad's embattled government and troops. Many of the rebel groups hit by the Russian strikes are also at war with the IS group.

Opposition activists said the Islamic State group took advantage of Russian airstrikes and the rebels' preoccupation with fighting Syrian army forces on other fronts in central Syria.

"At a time when the rebels are waging fierce battles against the Russian occupiers in Hama countryside, Daesh seizes the Infantry Academy and a number of villages in Aleppo," said Hadi Abdullah, an activist with close links to the Army of Conquest, a coalition that includes mainstream rebels as well as al-Qaida's affiliate, the Nusra Front. Daesh is an Arabic acronym for the IS group.

The Army of Conquest, which had made a string of advances against government troops in recent months, has come under attack from Russian warplanes.

Other activists wondered where the Americans were.

"Why didn't America attack Daesh fighters during their attack?" asked Rami Abdurrahman, director of the Britain-based Syrian Observatory for Human Rights.

The Observatory said the extremists' surprise advance north of Aleppo, which began Thursday night, is the most significant in months. It said the militants seized the villages of Tal Qrah, Tal Sousin and Kfar Qares north of Aleppo, Syria's largest city and its onetime commercial capital.

The group also seized a former army base known as the Infantry Academy that rebels captured from the Syrian army two years ago. The base is located around 10 miles (16 kilometers) northeast of Aleppo city and about one mile (1.6 kilometers) away from a government-held industrial zone on the northern edge of Aleppo.

The pro-IS Aamaq media outlet said the group seized six villages and other strategic positions in Aleppo province after battling with armed opposition groups there. It said the group opened its blitz with a surprise attack on the Infantry Academy, forcing rebels positioned there to withdraw after a number of their fighters were killed or captured.

It said militants then continued their advance toward nearby villages, killing and wounding rebels and seizing "qualitative weapons and ammunition" from them along the way.

It was not clear whether Hamedani's killing was related to that IS offensive. Abdurrahman said he was killed on the edge of the Kweiras military airbase, which the IS group has besieged for months. Syrian troops launched an offensive last week in an attempt to break the siege.

Kweiras base is about 31 miles (50 kilometers) away from the area of the IS offensive.

"Brig. Gen. Hamedani was martyred by Daesh terrorists during an advisory mission on the outskirts of Aleppo" on Thursday afternoon, said the terse statement read on IRNA.

Hamedani is one of the most senior Guard commanders to be killed in Syria, and the second to be killed this year. He was a veteran commander who had an important role in Iran's 1980-88 war with Iraq and was among top Iranian commanders coordinating fighting in Syria. Hamedani was also reportedly in charge of organizing and commanding the various Iranian-backed Shiite militias fighting alongside President Bashar Assad's forces.

Iranian officials, including Foreign Minister Mohammad Javad Zarif, lauded Hamedani's role in the fighting.

In January, Brig. Gen. Mohammad Ali Allahdadi was killed in an Israeli airstrike in Syria that also killed six Lebanese Hezbollah fighters.

Shiite powerhouse Iran is one of Syrian President Bashar Assad's main allies. Tehran has provided his government with military and political backing for years and has kept up its support since Syria's civil war began in 2011.

Guard commanders repeatedly have said Iran only has high-level advisers in Syria, denying it has fighters there.

Assad hails from Syria's Alawite religious minority, an offshoot of Shiite Islam. Syria's rebels mainly hail from the country's Sunni majority.

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Associated Press writers Nasser Karimi and Ali Akbar Dareini in Tehran, Iran, and Bassem Mroue in Beirut contributed to this report.