Germany has rejected a US appeal to send more troops to Afghanistan, amid signs of strain in the Nato mission.
Mr Jung has just visited German troops in Afghanistan
The US defence secretary had used a strongly worded letter to urge larger German deployment to south Afghanistan.
But his German counterpart, Franz Josef Jung, bluntly ruled out deploying any German soldiers to the area, which is at the heart of the Taleban insurgency.
In his letter, Mr Gates warned that without reinforcements the Nato effort could lose credibility in Afghanistan.
He urged Germany to consider a new mandate which could allow thousands more troops to be deployed.
But the German response was equally blunt.
Not only did Mr Jung reject the call for more combat troops to be sent to dangerous parts of southern Afghanistan, he also said Germany had no plans to change its force's deployment in the less violent north.
"We have agreed on a clear division of labour," said Mr Jung on Friday. "I think that we really must keep our focus on the north."
Germany currently has 3,200 troops stationed in northern Afghanistan and around the capital, Kabul.
According to the current parliamentary mandate, 3,500 troops can be sent to northern Afghanistan as part of Nato's 40,000-strong International Security Assistance Force (Isaf).
'Not up for discussion'
German Chancellor Angela Merkel also made clear that the mandate was "not up for discussion", her spokesman said.
There are concerns that the mission may be failing
German politicians are wary of making a greater commitment as opinion polls show public scepticism about the mission.
And Ernst Uhrlau, the head of Germany's foreign intelligence service (BND), has warned that the security situation in Afghanistan is expected to worsen in the coming months.
Correspondents say the German-US exchange comes amid growing signs of strain in the Isaf mission and in Nato as a whole.
HAVE YOUR SAY
Germany is a part of Nato and is obliged to send in more troops
Rob, Wirral, UK
The US has already promised to send an extra 3,000 marines - but is urging other Nato countries to do more amid rising Taleban attacks.
US Secretary of State Condoleezza Rice is due to fly to the UK next week for talks with Prime Minister Gordon Brown and Foreign Secretary David Miliband.
Nato's role in Afghanistan is expected to be high on the agenda.
So far, most Nato members have refused to send significant numbers to southern Afghanistan.
Canada's Prime Minister Stephen Harper has issued both the US and the UK with an ultimatum - that Canada will end its military mission in the dangerous south of Afghanistan unless other Nato countries send more reinforcements.
LEAD INTERNATIONAL FORCES IN AFGHANISTAN
Locations refer to International Security Assistance Force (Isaf)
Total contributing nations: 39
Isaf total strength: Approx 41,700