The German foreign ministry has warned about possible attacks on Istanbul's underground rail network and bus stops in the wake of Turkey's assault on Kurdish armed groups in northern Iraq.

"There could be increased attack activity by the PKK," the foreign ministry said in a statement on its website on Wednesday, referring to the Kurdistan Workers Party.

"Beyond that there are indications of possible attacks on the underground rail network and bus stops in Istanbul," the ministry added.

Opinion: What is next for Turkey's Kurdish peace process?

The warning comes as Turkey’s parliament held an extraordinary meeting on Wednesday, to discuss military operations against the Kurdish rebels and the Islamic State of Iraq and the Levant (ISIL), Anadolou news agency reported.

On Wednesday night, the Agence France Presse news agency also reported that the Turkish cabinet approved the plan allowing the US to use an air base in Incirlik. for attacks against ISIL.

Pro-Kurdish politicians have warned that Turkey's simultaneous attacks on PKK targets are threatening the peace negotiations with the Turkish government starting in 2012.

Al Jazeera's Bernard Smith, reporting from the majority Kurdish city of Diyarbakir, said Kurdish leaders have complained that they are "unfairly equated" with ISIL.

Our correspondent said that there are worries that intensified bombardments could "provoke" Kurdish armed groups to escalate retaliations.

"That could really cause problem in an already destabilised region," he said.

PKK targeted inside Turkey

Overnight, Turkish warplanes struck targets of the PKK in northern Iraq and in southeast region of Turkey, in a new wave of air raids against the Kurdish group, the prime minister's office said on Wednesday.

"Air operations were conducted throughout the night 28-29 July against the PKK terrorist group inside Turkey and outside," it said in a statement, listing six PKK locations in northern Iraq hit by the war planes.

It said shelters, depots, logistical bases and caves found to be used by the PKK were hit in the air strikes.

The latest operation is the first time that PKK fighters have been attacked inside Turkish territory.

The army has since Friday been carrying out daily air strikes against targets of the PKK in northern Iraq, as well as ISIL in Syria, in what the government calls a "war on terror" in response to a spate of attacks.

The operations came after a bombing last week blamed on ISIL that killed 32 mostly young Kurdish students in the Turkish town of Suruc on the border with Syria.

Turkey's President Recep Tayyip Erdogan said it is impossible to continue a peace process with Kurdish fighters and that politicians with links to "terrorist groups" should be stripped of their immunity from prosecution.

The PKK fought the Turkish state for over 30 years until a 2013 ceasefire was declared to make way for talks.

On Wednesday, Turkish foreign ministry undersecretary Feridun Sinirlioglu flew to Arbil in Iraq for meetings with Iraqi Kurdish leaders, a foreign ministry official said, in a clear bid to calm tensions.