For the PKK to target major cities such as Istanbul "is nothing new", Erdogan said after visiting the injured at an Istanbul hospital.
A car bomb targeted a bus shuttle service carrying anti-riot police as it was passing through the central Beyazit district in Istanbul, leaving 11 people including seven police dead.
Thirty-six people were injured.
"We will fight against terrorists relentlessly to the end," Erdogan said, adding that Turkey should be ready for attack at all times, according to AFP.
There was no immediate claim of responsibility for Tuesday's bombing, which occurred on the second day of the Muslim holy month of Ramadan.
Turkey is on high security alert following multiple attacks on its soil blamed on the PKK and the Daesh (ISIL) group.
Violence flared up last year between Kurdish rebels and government forces, shattering a 2013 ceasefire reached after secret talks between PKK leader Abdullah Ocalan and the Turkish state.
Over 40,000 people have been killed since the PKK took up arms in 1984 demanding an independent state for Kurds. Since then the group has narrowed its demands to greater autonomy and cultural rights.