KABUL (Reuters) - The international military mission in Afghanistan will fail if troop levels are reduced further, with potentially dangerous repercussions for the rest of the world, a delegation of U.S. lawmakers warned during a visit to Kabul on Monday.

Fifteen years after an American-led operation toppled the Taliban in response to the September 11, 2001, terrorist attacks, President Barack Obama is considering whether to maintain the current level of 9,800 U.S. troops or reduce it to 5,500 by the end of the year, as current plans call for.

"I cannot guarantee success if we keep 9,800, but I can ensure you failure if we go to 5,500," Republican Senator Lindsey Graham told reporters in Kabul.

"I will have a hard time supporting our continued presence here as it's not fair to those left behind... They just can't do the job. If we go to 5,500 this place will fall apart, quickly."

Josh Smith / Reuters U.S. Senator Lindsey Graham speaks to reporters in Kabul, Afghanistan July 4, 2016. REUTERS/Josh Smith

Graham joined U.S. senators John McCain, Benjamin Sasse, and Joe Donnelly in a visit timed so the bipartisan delegation could visit with troops during the Independence Day holiday.

The Obama administration should decide on troop levels "sooner rather than later," McCain said, arguing that reducing the number of troops could lead to a repeat of the disaster in Iraq, where Islamic State militants seized major cities and wide swathes of territory.

McCain, the Republican chairman of the Senate Armed Services Committee, sharply criticized the White House's decision last year to restrict U.S. forces from targeting Taliban fighters except in self-defense and other limited circumstances.

Those rules were recently relaxed on the recommendation of American commanders in Kabul, but McCain said it was "almost criminal" that the restrictions were in place for more than a year.

"The rules of engagement were so restrictive that it gave an advantage to the Taliban and other terrorist groups," the senator said.

The lawmakers highlighted Afghanistan's history as one of the original havens for al Qaeda terrorists and pleaded for more patience.

"Ultimately we're going to win this fight, it's just going to take decades," Sasse said. "The American people well understand that staying partnered with a good ally like the Afghan government is the best way to deny future safe havens to those who would plot jihadi attacks across the globe."

(Reporting by Josh Smith; Editing by Nick Macfie)