Six medical students were killed and 23 others wounded in a Taliban suicide attack at Afghanistan's main military hospital in Kabul on Saturday, the defence ministry said.

The blast took place in a tent used as a dining room by students at the city centre hospital, one of the biggest and best-equipped in Afghanistan.

The hospital is not used by Nato forces, though it is thought that foreign medics are frequently stationed there to train their Afghan counterparts.

"There was a suicide attack at a tent used as a dining facility by medical students inside the hospital," said defence ministry spokesman General Mohammad Zahir Azimi.

"The blast killed six people and wounded 23 others and all of them are medical students."

Responsibility for the blast was claimed by the Taliban, whose spokesman Zabihullah Mujahid said two attackers had entered the heavily-secured hospital.

A doctor inside the hospital, speaking to AFP on condition of anonymity by telephone, said people in the building he was in within the compound had been shut into one room as searches went on for other attackers but had now been evacuated.

Mohammad Zaher, head of Kabul police's criminal investigation department, said that searches were still going on at the hospital for another possible attacker.

However, Azimi denied reports that there was a second suicide attacker on the loose in the hospital.

Eyewitness Mohammad Hakim, who has a cart selling fruit nearby, described what had happened.

"It was a big explosion, I fell to the ground," he said. "The police arrived late at the scene, half an hour after the explosion took place."

The roads to the hospital were blocked and Afghan security forces including the spy agency's quick reaction forces cordoned off the facility, while police and soldiers were also on the scene.

Relatives of patients inside the hospital gathered nearby after trying but failing to find out if their family members are safe, an AFP reporter at the scene said.

Afghanistan's military are frequently targeted by the Taliban and other militants. There has been a recent spike in suicide bombings — a key Taliban tactic — and other attacks in Afghanistan.

Two rockets were fired overnight into Kabul city landing in an empty plot, causing no damage or casualties, interior ministry spokesman Zemarai Bashary told AFP.

Afghan forces are set to take increasing responsibility for security as foreign combat troops withdraw in a process starting from July but not due to be completed until 2014.

The worst fighting in Afghanistan usually comes in spring and summer and the fighting season is starting to get under way.

The Taliban announced the start of its spring fighting season at the end of last month.

On Thursday, 36 people were killed when militants attacked a road construction company's compound in eastern Afghanistan in the biggest attack since they announced the start of the fighting season.

There are around 130,000 international troops in Afghanistan fighting the Taliban and other insurgents.

The conflict started with the US-led invasion of Afghanistan in 2001 following the September 11 attacks and has now run for nearly ten years.