Parts of Afghanistan have seen a "spiral downwards" of violence, the incoming US regional commander said Monday, adding the fight against Taliban forces would continue unabated through the winter.

General David Petraeus told reporters outside Downing Street after talks with British Prime Minister Gordon Brown that there had been "significant progress" in some areas of the country.

But he added: "There's no question but that the trends in terms of violence and so forth have been in the wrong direction.

"So in that case, you're either spiralling downward or upward and in certain areas in Afghanistan clearly there has been a spiral downward that all involved... want to arrest and then get going back up again".

Petraeus, who was in charge of US-led forces in Iraq until earlier this year, takes charge of US forces in southwest Asia and the Middle East next month.

He also said he expected to see NATO troops fighting in Afghanistan "through the winter, perhaps a bit more than we have seen in the past."

Petraeus added: "I think we are going to endeavour to continue a higher level of operational tempo throughout so that there's not a lull in the fighting season that we continue...to take the fight to the enemy."

Asked whether US airstrikes in Pakistani territory were an important part of the fight, Petraeus again stressed that the country faced an "existential threat."

But he added that key figures in Islamabad including new President Asif Ali Zardari increasingly recognised the importance of that.

"This is a threat to Pakistan's very existence and it is one with which they must deal," Petraeus said.

His comments came after Pakistani and US troops last week exchanged fire along the Pakistani-Afghan border last week, after Washington said two US military helicopters came under fire.