More than half of Syria fell under the control of the Islamic State (ISIS/ISIL) after the jihadist group captured the ancient city of Palmyra, a monitor group said.

The U.K.-based Syrian Observatory for Human Rights (SOHR) told Reuters that ISIS seized Palmyra’s military air base and prison after advancing into the city and routing armed forces loyal to Syrian Dictator Bashar al-Assad and their allies on Wednesday.

A statement posted by ISIS followers on Twitter said the jihadist group was in full control of the city, including its military bases, adding that the fleeing pro-Assad troops left behind many of their dead.

Heavy clashes between the two sides resulted in the death of at least 100 Assad regime fighters, said the Observatory, which monitors the Syrian war through a network of sources on the ground.

In a rare public appearance on May 6, Assad admitted for the first time that his regime had suffered a spate of military defeats at the hands of Syrian rebels including ISIS.

U.S. Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, acknowledged on May 7 that Syrian rebel factions had made recent gains against Assad and suggested that the dictator should consider negotiating peace.

“The capture of Palmyra is the first time ISIS has taken control of a city directly from the Syrian army and allied forces, which have already lost ground in the northwest and south to other insurgent groups in recent weeks,” notes Al Arabiya News.

Gen. Jack Keane, former Vice Chief of Staff for the Army, told the Senate Armed Services Committee on Thursday that the U.S. was losing the war against ISIS, prompting criticism towards President Obama’s anti-ISIS strategy.

Keane himself said Obama’s efforts to fight ISIS are “fundamentally flawed.”

On March 11, Secretary of State John Kerry told lawmakers that “ISIL’s momentum has been diminished.”

Palmyra is home to a UNESCO World Heritage site that includes colonnaded streets, temples, and a theatre that have stood for nearly 2,000 years.

The ancient sites are at risk of being looted and destroyed if they have not been already.

On Thursday, the White House press secretary described ISIS’s seizure of Palmyra as a “setback” for the U.S.-led campaign against ISIS.

He added that President Obama disagrees with Republicans urging the deployment of U.S. ground troops to combat ISIS.

The global community must respond to ISIS, said French President Francois Hollande on Thursday.

“We have to act because there is a threat against these monuments which are part of humankind’s inheritance and at the same time we must act against Daesh,” declared Hollande, referring to ISIS by its Arabic name.

The Iraqi city of Ramadi, the capital of the country’s largest province Anbar, has also fallen to ISIS.