BERLIN — German troops are staying in the calmer northern regions of Afghanistan, the defence minister said Friday, despite pleas from Canada and the United States for more military muscle to help fight insurgents in the south.

"I have a clear mandate from the German parliament,'' German Defence Minister Franz Josef Jung told reporters Friday.

"It consists of 3,500 soldiers serving along the northern border and only helping out in the south for a limited period of time, as needed.''

Jung made the remarks in response to a letter from U.S. Defence Secretary Robert Gates seeking more substantial help in the volatile south of Afghanistan.

Troops from Canada, Britain, the Netherlands and the United States have borne the brunt of fighting the Taliban in the south, with support from Denmark, Romania, Estonia and non-NATO nation Australia.

Ottawa is looking to allies for 1,000 more troops and additional equipment for southern Afghanistan as Parliament debates whether to extend the Canadian military mission beyond February 2009.

Washington said it had no plans to send more U.S. troops beyond the decision last month to deploy 3,200 additional marines to Afghanistan, but promised to help Canada lobby other countries for reinforcements.

Gates has been trying to persuade NATO allies to contribute more to the International Security Assistance Force in Afghanistan, without much success.

According to Germany's Sueddeutsche Zeitung daily newspaper, the letter from Gates is 1-1/2 pages long and specifically asks Germany for helicopter units, infantry and paratroopers that could join the fight against Taliban militants in the south.

The refusal of Germany, along with France, Turkey and Italy, to send significantly more troops to the southern front lines has opened a rift within NATO. The issue is expected to feature prominently at a meeting of NATO defence ministers in Vilnius, Lithuania, next week.

In Ottawa, Gen. Rick Hillier, the chief of defence staff, said NATO has plenty of troops in Afghanistan but there just aren't enough deployed in the south where the need is greatest.

"There is no job outside of the south where you actually need extra troops right now,'' Hillier told reporters. "In fact, there is contemplation all the time in military circles of `Can you move troops from the rest of the country into the south where the need is most definite?'''

Most of Canada's 2,500 troops on the Afghan mission are in Kandahar province, birthplace of the extremist Taliban movement.

Hillier said Afghan President Hamid Karzai "has said Kandahar is the centre of gravity: as Kandahar goes, the rest of the country will go.''

"And therefore that's where the need is right now -- and the need is not in the north or the west or the northeast.''

But Germany has shown little inclination to reallocate its troops.

"If friends need help, then we will respond with support for a limited time -- as stipulated in our mandate,'' Jung said.

"But I think that our emphasis needs to remain in the north.''

There was a positive response Friday from another NATO ally.

The Belgian government approved the deployment of four F-16 warplanes to join NATO's force in Afghanistan. The planes would be sent in September to work with Dutch F-16s already in Afghanistan.

The deployment will increase the number of Belgian troops serving with the NATO force to 480, up from the current level of 360.

Belgium is also scheduled to send a training team to work with the Afghan army in October. However, the total number of Belgian troops in Afghanistan is due to fall to 260 by the end of the year as it withdraws other units, including troops helping to manage Kabul airport.

The Dutch are also scheduled to bring home two of their six F-16 fighter jets from Afghanistan later this year.

"It's going to be up to the individual states to make decisions about allocation of resources,'' U.S. State Department spokesman Sean McCormack told reporters in Washington.

"I won't make a secret of the fact that we are encouraging all of our NATO allies to do everything they can in terms of contributing resources.''

With files from The Canadian Press