The conflict in Afghanistan is deteriorating in some ways and will "not remain winnable indefinitely," the top US military commander in the country warned Thursday.

General Stanley McChrystal added that forces would be in a much stronger position once US President Barack Obama decides on troop levels in Afghanistan, while saying that it would be wrong to rush to make a decision.

"The situation is serious and I choose that word very, very carefully ... neither success nor failure can be taken for granted," said McChrystal, who has asked for up to 40,000 more troops to fight the Taliban.

"The situation is in some ways deteriorating but not in all ways," he told the International Institute for Strategic Studies (IISS) think-tank in London.