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KABUL: US Defence Secretary Robert Gates flew into Kabul on Monday on an unannounced visit, warning of "hard fighting" still ahead despite signs of progress in the NATO-led war against Taliban insurgents.

"There is no doubt there are positive developments going on, but I would say it's very early yet," Gates told reporters on his plane before landing in the Afghan capital.

He cautioned that there would be "some very hard fighting, very hard days ahead" as US, NATO and Afghan government forces step up pressure on Taliban militants in the south.

Gates acknowledged "bits and pieces of good news" when asked about the recent capture of senior Taliban leaders in neighboring Pakistan, but said it was probably too soon to say momentum had shifted to coalition forces.

"I think more needs to be done," he said, adding that a surge of US reinforcements was still in its initial stages.