The Bashar al-Assad regime launched airstrikes to help its purported enemy the Islamic State (ISIS/ISIL) advance in and around Aleppo, Syria’s largest city, the U.S. Embassy in Syria wrote on its official Twitter account.

Assad regime opposition officials echoed the U.S. Embassy’s accusations.

“Reports indicate that the regime is making airstrikes in support of #ISIL’s advance in #Aleppo aiding extremists against Syrian population,” Tweeted the embassy on June 1.

“With these latest reports, #Assad is not only avoiding #ISIL lines, but actively seeking to bolster their position,” also said the U.S. Embassy, later adding, “We have long seen that the #Assad regime avoids #ISIL lines, in complete contradiction to the regime’s claims to be fighting ISIL.”

On that same day, the embassy condemned the use of barrel bombs by the Assad forces in and around Aleppo, which ended up killing at least 70 civilians, including women and children. Aleppo is the capital city of a province that shares its name, the most populous in Syria.

“The fact is there is no better recruiting tool for #ISIL that the brutality of the #Assad regime,” it also tweeted.

Echoing the U.S. Embassy, Khaled Khoja, the president of the main Syrian exile opposition group, accused the Assad regime of using its warplanes “as an air force for ISIS,” reports The New York Times.

“It was never this blatant,” Abu Abdo Salabman, a spokesman for the Sham Revolutionary Brigades, an anti-ISIS rebel group, told The Times, referring to active coordination between the Syrian regime and ISIS.

“There is clear advanced coordination this time, and not just a side trying to take advantage of the other,” added Mr. Salabman, who uses a nom de guerre out of fear for his safety.

Abu Yusuf, a spokesman for the Shamiyeh Front, another rebel group fighting ISIS, told The Times that the regime troops in Syria are targeting territory in and around Aleppo city that ISIS has long been trying to capture.

Mr. Salabman said that Syrian government airstrikes are only targeting non-ISIS rebels.

“Neither American officials nor Syrian insurgents have provided proof of such direct coordination, though it has long been alleged by the insurgents,” notes The Times.

On Tuesday, Mari Harf, a spokeswoman for the State Department, told reporters that U.S. officials were looking into the allegations but added that the U.S. does not have “independent evidence” to back the claims.

Syrian military officials have denied helping ISIS fight against rival jihadists, dismissing the accusations as “nonsense” claims by the U.S. and Syrian opposition activists, reports Reuters.

“The Syrian army is fighting Islamic State in all areas where it is present in Syria,” a military source told Reuters.

Although it suspended its operations in 2012, the U.S. Embassy in the Syrian capital of Damascus continues to publish messages on its official Twitter feed.

Also on June 1, the embassy tweeted that Syrian dictator Assad “lost legitimacy long ago and will never be an effective counterterrorism partner.”

The Assad regime considers all Syrian opposition insurgents as foreign-backed “terrorist organizations,” notes Reuters.

SANA, a state news agency, on Tuesday claimed that the Assad armed forces had “eliminated” various jihadists in Aleppo, adding that airstrikes had taken out some of the group’s military equipment.

According to the Syrian Observatory for Human Rights, which monitors the Syrian war using a network of sources on the ground, the Assad military recently carried out airstrikes in the Aleppo province, including inside the capital city and on the ISIS-held town of al-Bab.

The Observatory reported that ISIS had beheaded several rebels in Aleppo.

Some rebels have questioned why the U.S.-led air campaign against ISIS in Iraq and Syria is not targeting the jihadists in and around Syria’s largest city of Aleppo.

“The United States has resisted calls for increased assistance to the rebel coalition because it is a muddle of groups including, most notably, the Qaeda-linked Nusra Front, even though the United States is seeking to recruit some of those insurgents to help it battle the Islamic State,” reports The New York Times.