Defense Secretary Jim Mattis and Secretary of State Rex Tillerson testify about authorizations for the use of military force before the Senate Foreign Relations Committee on Oct. 30. (Jonathan Ernst/Reuters)

Two senior members of President Trump’s national security team told lawmakers Monday that they believe they have the legal authority to conduct operations against terrorist groups, including the Islamic State, and said there was no need for a new war authorization to replace the one passed immediately after the Sept. 11, 2001, attacks.

Secretary of State Rex Tillerson and Defense Secretary Jim Mattis also told the Senate Foreign Relations Committee that any attempt to place time limits or geographical constraints in a new Authorization for the Use of Military Force could cripple efforts to fight terrorists.

Some lawmakers have said Congress needs to update laws that have provided authority to fight terrorist groups and detain militants including al-Qaeda and the Islamic State on multiple continents.

If Congress decides to go ahead and write an updated law, Tillerson and Mattis said, lawmakers should not constrain the military’s ability to go after terrorists anywhere in the world.

"The collapse of ISIS's so-called caliphate in Iraq and Syria means it will attempt to burrow into new countries and find safe havens," said Tillerson, using an acronym for the Islamic State. "Our legal authorities for heading off a transnational threat like ISIS cannot be constrained by geographic boundaries. Otherwise, ISIS may reestablish itself and gain strength in vulnerable spaces."

Mattis told the committee that time restrictions would also undermine combat operations.

"We cannot put a firm timeline on conflict against an adaptive enemy who could hope that we haven't the will to fight as long as necessary," said Mattis, who also stated that the Pentagon is shifting the military's counterterrorism strategy to focus more on Africa.

Lawmakers mentioned the possibility of using military force in crises involving North Korea, Iran and Venezuela, as well as the ongoing efforts against militant groups that “change their name as often as a rock-and-roll band,” Mattis said.

The hearing was propelled by the killing of four U.S. troops in Niger on Oct. 4 in an apparent ambush as they patrolled in an area with groups loyal to both al-Qaeda and the Islamic State.

Some members of Congress were taken aback by the size and scope of U.S. combat forces deployed throughout Africa. About 800 Americans are based in Niger to run counterterrorism operations and train and advise local troops, and hundreds more U.S. forces are in other African countries.

Laws enacted in 2001 and 2002 gave the military the legal authority to fight international terrorism. Even though the United States is fighting groups that didn’t exist back then, including the Islamic State, the legal basis remains sound, Mattis said. He urged Congress not to repeal existing law, at least without a new authorization already in place.

“The uncertainty accompanying that situation could only signal to our enemies and our friends that we are backing away from this fight,” he said. “It would stall our operations, immediately reduce allied commitments and support, and create significant opportunities for our enemies to seize the initiative.”

A number of lawmakers said it is time to update the law to be more consistent with current conditions and threats. Sen. Tim Kaine (D-Va.) and Sen. Jeff Flake (R-Ariz.) are co-sponsoring a proposal providing new legal authority to combat al-Qaeda, the Taliban and the Islamic State.

“This is one of the most important topics the United States Senate, and this committee, could ever consider: Under what circumstances, and legal authorities, should the United States send our men and women into war?” said Sen. Benjamin L. Cardin (Md.), the top Democrat on the committee.

Flake noted that not a single member of the committee was in the Senate when the original war authorizations were signed by President George W. Bush more than a decade ago.

“Congress needs to weigh in,” he said. “We have to make sure our adversaries, and our allies, and most importantly, our troops know that we speak with one voice.”

Shortly before Tillerson and Mattis started testifying, a small group of protesters in the room chanted “Stop endless war,” before they were evicted.

“This has been a 16-year struggle,” said Sen. Ron Johnson (R-Wis.). “I don’t think it’s going to be over any time soon.”