U.S. President Barack Obama’s announcement last week that a small special operations force would assist rebels fighting Islamic State militants and the Assad regime in Syria has prompted several lawmakers in Congress to call for a new authorization of military force. Obama previously vowed that no American boots would touch the ground in the U.S. fight against ISIS, and that intervention would be limited to airstrikes, said members of Congress concerned over the new plan.

“The administration’s announcement that it will deploy special operations forces into Syria to combat ISIL marks a major shift in U.S. policy,” said Sen. Brian Schatz, D-Hawaii, a member of the Senate Foreign Relations Committee, the Washington Post reported Sunday. “This shift in policy is a strategic mistake,” Schatz said, added that federal law stipulates that Congress first authorize Obama’s escalation of military involvement in Syria.

While other Democrats expressed concern about an AUMF, some Republicans, like Sen. Tom Cotton of Arkansas, said the president’s new plans for Syria do not go far enough. The Foreign Relations Committee was preparing for a detailed briefing from the White House on the Syria operation this week, an aide to committee Chairman Bob Corker, R-Tenn., told the Post. Both parties have been critical of Obama’s ISIS strategy in Syria and Iraq, particularly after Russia intervened by launching its own airstrikes against militants and U.S.-trained Syrian forces showed little potency against terror groups.

In July, Obama authorized the use of U.S. air power to assist U.S.-trained rebels, if they were attacked by ISIS militants or the Assad regime, according to a Bloomberg News report. That constituted broader U.S. rules of engagement, some lawmakers said.

Obama, who had ruled out sending ground troops into Syria or Iraq, counted on defeating ISIS with airstrikes. The U.S. and its allies launched more than 5,000 airstrikes over 12 months in Iraq and Syria against ISIS, according to Bloomberg News’ report in August.

Meanwhile, Congress has not resolved party differences over what a new AUMF would look like. The Senate failed to approve an ISIS-related AUMF in 2014.

The White House said Friday that fewer than 50 special operations soldiers would go into Syria, in a noncombat advisory role. But many in Congress questioned whether the force would not have combat functions.