Syrian government aircraft bombed Iraqi military targets held by the Sunni militant group Islamic State of Iraq and Syria, or ISIS, even as the first batch of 90 U.S. military advisers landed in Iraq Tuesday.

The latest airstrikes were conducted in the western province of Anbar, which is the largest governorate in Iraq geographically and shares a border with Syria, Jordan and Saudi Arabia. And while Iraqi state media had initially reported that the attacking aircraft were U.S. drones, Washington denied the claims and a U.S. government official told ABC News that the strikes were believed to be conducted by Syria.

The American official said that the U.S. government has got “information that the Syrians are behind the fighter aircraft bombing in western Al Anbar,” adding that he was not sure whether the Iraqi government had requested Syria to attack, according to ABC News.

“Press reports that U.S. drones struck ISIL [ISIS] targets in Iraq today are false.  No such action has been taken,” Navy Rear Adm.  John Kirby, Pentagon’s press secretary, reportedly told the media, according to ABC News. In a separate briefing, he announced that U.S. troops landed in Baghdad Tuesday.

Iraq’s Prime Minister Nouri al-Maliki had formally requested the U.S. last week to conduct airstrikes on nearly 10,000 ISIS militants -- 7,000 in Syria and 3,000 in Iraq. However, President Barack Obama's administration decided against using airstrikes to combat ISIS, and instead sent military advisers to help Iraqi government troops halt the militant's group's advance. ISIS, in a relatively short span of a few days, has taken over vast swathes of Iraq's north and now controls the country's western border.

The U.S. Department of Defense said, in a statement, that it will send four teams of 50 people each, to Iraq in the coming days, to assess the current situation on the ground and provide recommendations to government troops on how to fight ISIS.

The U.S. government will also fly nearly 35 surveillance flights over Iraq daily to gather information of ISIS’ movements and attacks, according to ABC News.

“We expect that they’ll start to flow their assessments up through the chain of command in about two to three weeks,” Kirby said in a statement Tuesday, adding that ISIS is “still a legitimate threat to Baghdad.”