A Kurdish militant group linked to the PKK terrorist organization has claimed responsibility for an attack at Istanbul's second airport this week that left an aircraft cleaner dead and injured another.In a statement, the Kurdistan Freedom Hawks (TAK) said they carried out the mortar attack on Wednesday at Sabiha Gökçen airport that also damaged five aircraft.The group has in the past claimed responsibility for some terrorist attacks, such as a 2012 assault on a Turkish military bus that killed two soldiers and injured 12 people in a coastal resort town.Both TAK and PKK are regarded as terrorist organizations by Ankara and the United States.The PKK stepped up its campaign of violence, especially in southeastern Turkey since the summer, killing dozens of troops, police officers and civilians. Along with the PKK, the Revolutionary People's Liberation Party-Front (DHKP-C) and similar ultra-left terrorist organizations are active in Turkey, though their attacks are confined to assassinations and minor bomb attacks.