KABUL- At least 25 people were killed and more than 50 wounded in an attack on Afghanistan's biggest military hospital on Tuesday which saw two heavy blasts followed up by gunmen assaulting the site in central Kabul, officials said.

The explosions took place at the entrance of the 400-bed Sardar Mohammad Daud Khan hospital and were followed immediately with an assault by a group of gunmen, Taliban spokesman Bilal Karimi said.

Four of the attackers were killed by Taliban security forces and a fifth was captured, he said.

The blasts add to a growing list of attacks and killings since the Taliban completed their victory over the Western-backed government in August, undermining their claim to have restored security to Afghanistan after decades of war.

There was no immediate claim of responsibility but the operation was typical of the complex attacks mounted by Islamic State. It follows a string of bombings by the group which has emerged as the biggest threat to Taliban control of Afghanistan.

A Taliban security official, speaking on condition of anonymity, said at least 25 people had been killed and more than 50 wounded in the assault but there was no officially confirmed casualty toll.

Photographs shared by residents showed a plume of smoke over the area of the blasts near the former diplomatic zone in the Wazir Akbar Khan area of the city and witnesses said at least two helicopters flew over the area as the assault went on.

A health worker at the hospital, who managed to escape, said he heard a large explosion followed by a couple of minutes of gunfire. About ten minutes later, there was a second, larger explosion, he said.

He said it was unclear whether the blasts and the gunfire were inside the sprawling hospital complex.

Islamic State, which has carried out a series of attacks on mosques and other targets since the Taliban's seizure of Kabul in August, mounted a complex attack on the hospital in 2017, killing more than 30 people.

The group's attacks have caused mounting worries outside Afghanistan about the potential for the country to become a haven for militant groups as it was when an al Qaeda group attacked the United States in 2001.

The concern has been worsened by a spiralling economic crisis that has threatened millions with poverty as winter approaches and left thousands of former fighters with no employment.

The abrupt withdrawal of international support following the Taliban victory has brought Afghanistan's fragile economy to the brink of collapse just as a severe drought has threatened millions with hunger.

(Additional reporting by Jibran Ahmad in Peshawar, Islamabad newsroom; Writing by James Mackenzie; Editing by Andrew Heavens and Nick Macfie) ((james.mackenzie@thomsonreuters.com; +39 02 66129533 ;))