Nato’s mission in Afghanistan was rocked by another blazing row over the refusal of some coalition members to fight the Taleban in the south.

An unusually stern letter from Robert Gates, the US Defence Secretary, to his German counterpart about the role of Germany’s troops in Afghanistan caused anger not just in Berlin but elsewhere in the alliance.

Washington has taken the lead in putting pressure on Nato with a warning that the credibility of the alliance is at stake. But Mr Gates’s latest intervention seems likely to cause more division.

His letter to Franz Josef Jung, the German Defence Minister, went to the heart of the problem that has faced Nato since its mission expanded throughout Afghanistan, and in particular to the southern provinces where the Taleban are concentrated.

This is the question of national caveats under which countries such as Germany, whose troops are based in the north, refuse to deploy elsewhere without authorisation from their governments.

The only Nato members that have no national caveats are the ones fighting the Taleban in the southern provinces of Helmand, Kandahar and Uruzgan and also in eastern Afghanistan. It was one of the conditions for agreeing to serve in the south.

The countries are the US, Britain, Canada, the Netherlands and Denmark, who provide the majority of the fighting troops. Other more limited troop contributors in the south include a small but effective Estonian unit and headquarters staff from Slovakia and Romania. The Americans and Poles are also engaged in combat in eastern Afghanistan.

Elsewhere in Afghanistan the role of every other Nato country is covered by caveats that range from a ban on deploying out of area — except in the most extreme circumstances — to no night flying, largely because of lack of technical capability; no flying in poor weather; no involvement in riot control and no venturing from bases without the maximum force protection or too far from the nearest hospital.

Mr Gates’s written demands for German troops to be sent to the south provoked an instant putdown from Berlin, which reminded Washington that Germany’s remit in Afghanistan was to defend the north, and that it had 3,100 troops there.

Mr Jung made it clear yesterday that Germany had no plans to deploy troops to the south, where most of the fighting has been going on since the early summer of 2006.

German diplomatic sources said the letter from Mr Gates had been harsh, although they would not divulge the contents. Mr Jung replied in similar mode with a “direct and stern” letter to Mr Gates, according to Suddeutsche Zeitung, a German newspaper.

The whole question of burden-sharing in Afghanistan — in particular sharing the burden of combat — is due to come up at Nato’s next summit in Bucharest in April. One Nato diplomat said: “I think Mr Gates’s intervention is more about domestic politics than anything else but sometimes I wonder whether the US realises the negative impact these spats have outside America.”

The Americans are sending another 3,200 troops to Afghanistan to boost numbers because of the failure of other countries to offer more soldiers, and Mr Gates has had to justify this decision before Congress. Pentagon officials have been appearing at Senate hearings in recent days.

German defence sources said that in emergencies where help was urgently needed in the south, Germany would consider sending troops from the north but only for a brief period.

“The location we have been given is in the north and that is where we are staying and, by the way, although it might seem more peaceful at the moment, there is no knowing whether that will last,” one German source said.

Mr Gates has also raised the issue of sending troops to the south with Hervé Morin, the French Defence Minister, at a meeting in Washington. France has about 1,600 troops in Afghanistan, most of them serving in Kabul.

Germany, France, Italy and Spain — the latter two countries with troops in western Afghanistan — all agreed at the Nato summit in the Latvian capital of Riga last year to send troops to the south, but only in extremis.

Since the summit, no troops from these countries have gone to help the Nato countries in the south.

Nato commanders say the operation in Afghanistan needs another 7,500 troops. There are about 41,000 serving with Nato’s International Security Assistance Force.

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