The Taliban are killing hundreds of Afghan army and police forces each month. The New York Times said the death toll is making it harder to recruit new soldiers.

"So far, they have taken hard casualties over the last year," Mattis told reporters at the Pentagon.

"But they have continued to fight, and we are adjusting tactics. We're bringing more support in certain areas," he added, declining to go into further detail.

In terms of overall violence -- impacting soldiers and the citizenry alike -- the Afghan conflict could overtake Syria as the deadliest conflict in the world this year, analysts say.

The grim assessment suggests President Donald Trump's much-vaunted strategy for Afghanistan is -- like those of his predecessors -- failing to move the needle on the battlefield.