Afghan forces have been battling Taliban fighters to retake the city of Kunduz, a day after it fell to the insurgents in their biggest victory since their removal from power in 2001.

Heavy fighting has been reported, and President Ashraf Ghani said his forces had regained some government buildings as fresh troops have been arriving to support the offensive.

The Taliban had forced Afghan troops and officials to retreat to the airport and freed hundreds from jail on Monday.

The attack on the northern city came as President Ashraf Ghani completed his first year in office.

In a televised address on Tuesday, he said "progress" was being made recapturing Kunduz but security forces had been hampered by the Taliban using civilians as human shields.

"The government of Afghanistan is a responsible government and it cannot bomb its people and compatriots inside the city and will not do so," he said.

Heavy casualties had been inflicted on "the enemy", he added, although this was denied by the Taliban.

US military planes have supported the operation, striking Taliban positions on the outskirts of the city, a Nato spokesman said.

Kunduz is the first provincial capital seized by the Taliban since they lost power in the US-led invasion 14 years ago.