Three German soldiers have been killed after coming under attack in northern Afghanistan, Germany's defence ministry has said.

The soldiers were conducting a joint operation with Afghan forces near the northern city of Kunduz on Tuesday when unidentified assailants opened fire on their patrol, ministry officials said.

The men fired back and requested air support and reinforcements.

Germany has about 3,700 troops serving with the Nato-led International Security Assistance Force (Isaf), stationed in the north of the country.

There has been a rise in violence across Afghanistan since campaigning for the country's presidential election officially began last week.

The poll is due to be held on August 20.

Forty-one candidates are standing for the five-year term, including Hamid Karzai, the incumbent president.