Hagel, confirming a change in the U.S. drawdown schedule first reported by Reuters in November, said the additional forces were needed because delays in signing security pacts had impacted plans to raise troops from other countries.

However, he also said that a particularly violent surge of Taliban attacks in Kabul in the last two weeks was a reminder of the continued need for a foreign presence.

"The recent wave of Taliban attacks has made it clear that the international community must not waver in its support for a stable, secure and prosperous Afghanistan," said Hagel, who arrived in Kabul unannounced on Saturday morning.

The NATO-led combat mission in Afghanistan officially ends in two weeks with a sharp reduction in western forces, but troops that stay behind will still provide "combat enabler" support to Afghan soldiers, Hagel said.