The Airport Security Force Academy in the compound of Pakistan's international airport in Karachi is under attack by unknown armed men.
The attack follows an all-night siege at the airport that resulted in the deaths of at least 36 people.
Sources told Al Jazeera that intense firing was taking place between security forces and attackers.
Al Jazeera's Kamal Hyder said that that four or five men had attacked the academy within the compound of the airport, adding that all activities at the airport had been suspended.
"There are a number of people wounded and we are waiting for more reports," Hyder said, adding that the armed men were carrying water and food supplies.
"After the split [within Taliban], Pakistani Taliban is trying to show that it has not weakened through these attacks. And they should be taken seriously by the government"
"The airport was declared open yesterday at 10GMT following Monday's attacks," Hyder said, stressing that the attack came only hours after the government declared the airport safe.
The Pakistani Taliban claimed responsibility for the assault and gave warning that more attacks were on the way.
At least two dozen people were wounded and flights were suspended as a result of Monday's attack.