KABUL, March 8 (Xinhua) -- The counter-attack operation in a military hospital in Afghanistan's capital of Kabul ended on Wednesday, after all gunmen were shot dead by security forces, said a security source.

"The Afghan commandos clearance operation against gunmen now ended at around 03:30 p.m. local time after they had managed to eliminate all the gunmen," the source told Xinhua anonymously.

The attack came in the morning after one suicide bomber struck the back gate of Sardar Mohammad Daud Khan Hospital in fortified Wazir Akbar Khan diplomatic district.

At least 30 people were killed and over 50 others wounded in the deadly attack, according to initial information provided by sources.

The initial blast enabled four gunmen holding grenades and disguised as medical staff to enter the main building. They opened indiscriminate fire on patients and medical staff, according to security sources.

"The Afghan National Security Forces investigation teams are at the scene to assess the casualties at the military hospital as a result of today's terrorist attack," Interior Ministry Spokesman Sediq Sediqi twitted.

An Afghan army helicopter dropped Afghan commandos to the top of the building shortly after the clashes.

Further details about the incident are still forthcoming amid the absence of any official statement.

The Islamic State (IS) group has claimed responsibility for the attack. The Amaq, an IS so-called news agency, said in an online Arabic statement that the IS fighters had killed over 100 people in the attack.

Wednesday's terrorist attack is the second assault in the fortified 400-bed hospital, which is located in the diplomatic district near the U.S. embassy.

In mid-2011, some six medical students were killed and several others wounded in a suicide bombing.

On March 1, some 23 people were killed and 119 others wounded in two coordinated attacks in Kabul, which has been claimed by the Taliban insurgents.