The United States remains in armed conflict with al-Qaida and its affiliates, a fight likely to last a decade or two, senior Pentagon officials told Congress on Thursday in arguing against changes to the 2001 military force law used in the war on terror.

Acting General Counsel Robert Taylor and Michael Sheehan, an assistant secretary of defense for special operations, said the authorization for the use of military force is an effective law and expressed concerns that any congressional revisions would restrict combatant commanders.

The law "suits us very well," Sheehan told the Senate Armed Services Committee.

Republicans, Democrats and civil libertarians expressed serious concerns that the law — enacted just days after the Sept. 11, 2001 terrorist attacks — amounts to a blank check for the president. More than a decade after the law, President Barack Obama has used it to target terrorists worldwide with fatal drone strikes, including on Americans overseas.

The law gave President George W. Bush the authority to launch the invasion of Afghanistan and target al-Qaida, saying the commander in chief has the authority to attack "nations, organizations or persons he determines planned, authorized, committed or aided the terrorist attacks that occurred on Sept. 11, 2001, or harbored such organizations or persons, in order to prevent any future acts of international terrorism against the United States by such nations, organizations or persons."

The issue came to the forefront earlier this year as Sen. Rand Paul, R-Kentucky, waged a nearly 13-hour filibuster of John Brennan's nomination for CIA director over the president's authority to use drones in the United States. The Senate eventually confirmed Brennan.

Lawmakers have argued for changes to the law to deal with emerging threats and ensure greater congressional oversight. Sen. John McCain, a leading Republican, said the authority in the law is out of proportion and no longer applies.

"Basically you got carte blanche what you're doing around the world," McCain told the witnesses.

Sen. Angus King, an independent, told the Pentagon officials that they've "essentially rewritten the Constitution," usurping the Congress' power to declare war.

Questioned about how long the fight will go on, Sheehan said it would last beyond Obama's second term.

It is "going to go on quite a while, at least 10 to 20 years," he said.