Marine Lt. Gen. Kenneth McKenzie warned the Senate Armed Services Committee against an abrupt withdrawal of American forces or change in strategy despite frustration over the status of the 17-year conflict. He said he doesn't know how long it will take to develop an Afghan force capable of defending its own country.

"If we left precipitously right now, I do not believe they would be able to successfully defend their country," McKenzie said.

McKenzie said the U.S. and its allies need to keep helping the Afghans recruit and train forces to fight the Taliban's estimated 60,000 troops.

"They're fighting hard, but their losses are not going to be sustainable unless we correct this problem," McKenzie said, in one of the U.S. military's more dire assessments of the Afghan losses.

In its most recent report to Congress, in October, the special inspector general for Afghanistan said Afghan casualty numbers had been reported only in classified form since September 2017 at the request of the Afghan government. The report, however, said that the average number of casualties between May and October this year was the greatest it has ever been during similar periods.