Mattis, several members of the White House and representatives of the military command oppose the move because they believe that this step could plunge the United States in a confrontation with Iran which itself could lead to a retaliatory strike against the US military deployed in Iraq and Syria.
The defense secretary, Chairman of the Joint Chiefs Gen. Joseph Dunford and Special Presidential Envoy for the US-led anti-Daesh (terror group, banned in Russia) coalition Brett McGurk instead advocate focusing on pushing the militants out of the Syrian Raqqa, the media reported.
The US-led coalition of 69 members is conducting airstrikes, ground-based and rocket-propelled artillery fire against Daesh in Syria and Iraq. The strikes in Iraq are conducted in support of the Iraqi government, but those in Syria are not authorized by the UN Security Council or the government of President Bashar Assad.