Gunmen dressed as doctors attacked a military hospital close to the U.S. embassy in the Afghan capital, Kabul, on Wednesday and battled security forces inside the building for several hours, officials and witnesses said.

The attack began when a suicide bomber blew himself up at the rear of the 400-bed Sardar Mohammad Daud Khan hospital and three attackers with automatic weapons and hand grenades entered the complex, a security official said.

The gunmen, dressed as medical personnel, had taken position on upper floors of the hospital and engaged special forces sent to the scene, officials said.

Security forces blocked off the area around the hospital, near a busy traffic intersection. As fighting went on, a second explosion was heard from inside.

"Our forces are there and there is heavy fighting," said defence ministry spokesman Dawlat Waziri. He said one attacker had been killed and another two were holding out, while one soldier had been killed and three wounded.

A spokesman for the public health ministry said at least two dead and 12 wounded had been taken to other hospitals.

The attack on the hospital, across the road from the heavily fortified U.S. embassy, underlines warnings by government officials that high-profile attacks in Kabul are likely to escalate this year as the Taliban step up their insurgency.

"There is an ongoing terrorist attack in a hospital which tramples all human values," President Ashraf Ghani said in impromptu remarks during a speech for International Women's Day.

"In all religions, a hospital is regarded as an immune site and attacking it is attacking the whole of Afghanistan."

The NATO-led Resolute Support mission said it was ready to assist Afghan security services.

One worker at the hospital, Abdul Qadir, told Reuters that he saw one gunman dressed in a white doctor's coat take out a concealed AK-47 assault rifle and open fire, killing at least one patient and one hospital worker.

He also heard firing from several other parts of the hospital, which treats military casualties from around Afghanistan.    

The attack comes just a week after dozens were killed and wounded in coordinated attacks on a police station and an office of the intelligence service in Kabul.

That attack was claimed by the Taliban, who are seeking to expel foreign troops, defeat the U.S.-backed government and reimpose Islamic law after their 2001 ouster.

There was no immediate claim of responsibility. A Taliban spokesman said he had no immediate information.

Away from Kabul, dozens of people have been killed over the past few days in fighting across Afghanistan, from Kunduz and Baghlan in the north to Farah in the southwest and Helmand and Kandahar on the Pakistan border in the south.