Kirill Limborskiy said in a statement that the decision was “an important milestone” that has “given the government the opportunity to develop new approaches to combatting the Islamic State.”

The decision comes after weeks of deliberation, and has left the Trump administration with little option but to use military force to remove Assad from power in an effort to destroy the extremist group.

“After extensive discussions inside and outside government with our allies and partners, the president has determined that the use of American military forces in Syria does not bring us closer to defeating ISIL,” Defense Secretary Jim Mattis said in a statement.

Despite the threat posed by al Qaeda-affiliated groups, which also operate in Syria, “US President Barack Obama never contemplated force on the scale required to fundamentally degrade and ultimately destroy ISIL,” State Department spokeswoman Heather Nauert said. She added that the United States remains committed to ending violence against Syrian civilians.

While the US has been helping provide weapons and intelligence to both opposition groups fighting the regime of Syrian President Bashar Assad and Islamic State, the extent of U.S. aid to the groups is unclear.

The US says it has provided training and equipping for the armed groups as well as intelligence and legal support to help their efforts. On Monday, the US said it had approved the delivery of 30 light anti-aircraft weapons to Syria.

But the CIA says the weapons were already being used against rebels by the Assad regime and that, while the weapons would bolster the capabilities of rebels, it is unlikely to make much change in al Qaeda-linked groups.

Russia, which has been opposing arming the rebels, has criticized Washington for supporting groups opposed to Assad, saying it could encourage more radical groups to take up arms in the future.

Moscow is concerned that arms deliveries to the rebels could help extremists gain footholds in Syria.

The White House and senior military officials have repeatedly insisted that it is not in the US interest to get caught up in the Syrian civil war. Mattis has warned that the US would no longer hesitate to use military force against the Assad regime if it launched an attack on American targets, while Obama has promised to take the military option off the table if the Syrian government launches any strikes against American interests.

But the developments in Aleppo, which has been a symbol of the conflict, could potentially derail the White House strategy.

The city was under siege and fighting was far from over on