BERLIN, Nov 7 (Reuters) - U.S. President Barack Obama's national security adviser, James Jones, has said there is no guarantee that sending extra troops to Afghanistan would solve NATO's problems, and that they could just be "swallowed up". In an interview published on Saturday, the German weekly magazine Der Spiegel asked Jones whether he agreed with General Stanley McChrystal, the top U.S. and NATO commander in Afghanistan, that an increase of about 40,000 troops was needed.

"Generals always ask for more troops," Jones said.

"I believe we will not solve the problem with troops alone. The minimum number is important, of course. But there is no maximum number, however.

"You can keep on putting troops in, and you could have 200,000 troops there and Afghanistan will swallow them up as it has done in the past," he said, according to comments published by the magazine's website in English and German.

After weeks of internal deliberations, Obama's advisers are believed to be moving towards a hybrid strategy that would combine greater protection for population centres with more drone and special operations strikes against the Taliban.

The leading options under consideration would add at least 10,000 to 15,000 U.S. troops, but an announcement is expected to be weeks away.

Jones was asked how much longer U.S. forces would remain in Afghanistan, an area of foreign policy that has come under increased scrutiny since the revelation that the Afghan presidential vote in August was heavily tainted by fraud.

"I don't know how long," he said. "But I know our president and other heads of government are pressing for everything to be done to ensure the Afghans assume responsibility."

As of late October, Washington had 67,000 troops in Afghanistan. Other nations, mainly NATO allies of the U.S., have some 42,000 troops in the country.

Jones said terrorist networks were continuing to develop in neighbouring Pakistan.

"The danger from there is growing," he said. (Writing by Dave Graham; Editing by Kevin Liffey)

AlertNet news is provided by