Afghan forces have begun an operation to retake the city of Kunduz, after suffering a major military reverse when it was seized by the Taliban on Monday.

The president said security forces were "retaking government buildings" and inflicting "heavy casualties".

The US said it had carried out an air strike on the city early on Tuesday.

The Taliban had forced Afghan troops and officials to retreat to the airport and freed hundreds from jail, in one of their biggest assaults since 2001.

The attack on the northern city came as President Ashraf Ghani completed a year in office. In a televised address on Tuesday, he vowed to retake Kunduz and accused the Taliban of "using civilians as 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.

Kunduz is the first provincial capital seized by the Taliban since they lost power in the US-led invasion 14 years ago, and the Taliban's advance there will pile pressure on Mr Ghani's unity government.

It said it had deployed hundreds of fresh troops to help with the effort to retake the city on Tuesday.

Heavy fighting was reported, and officials said security forces had regained control of the area around the provincial prison and the police headquarters.

An eyewitness in Kunduz told the BBC that Taliban reinforcements arrived on Tuesday, with the situation too dangerous for locals to leave.

The Taliban's new leader, Mullah Akhtar Mansour, said the government should admit defeat.

Kunduz residents "should not be worried about their lives and property" and should "continue as normal", he said in a statement.

But the eyewitness said that while some locals had welcomed the Taliban, "most people are out there (on the streets) out of fear, and not out of sympathy".

A spokesman for the Nato-led coalition in Afghanistan said US forces had conducted an air strike "to eliminate a threat to coalition and Afghan forces" in the area.

Nato ended its combat mission in Afghanistan in December, withdrawing most of its troops apart from a 13,000-strong residual force used for training and counter-terrorism operations.

Kunduz is strategically important as a transport hub for the north of the country.

The insurgent attack began at dawn on Monday as fighters fell upon the city from different directions. They captured key buildings and freed about 500 prisoners, including members of the Taliban, from the jail.

Officials and troops retreated to the airport, though dozens of civilians who fled to the airport were reportedly turned away by security forces.

Medical charity Medecins Sans Frontieres‎ said it was treating more than 100 people wounded in heavy fighting, and its facility in the city was full.

Militant violence has increased across Afghanistan since the departure of most US and Nato forces last year.

Kunduz province has seen a number of attacks since April, with the Taliban joining forces with other insurgents.