TEHRAN - Iran issued an advisory warning its citizens Saturday to avoid land travel to neighbouring Turkey after an Iranian bus was attacked by gunmen.

No one claimed responsibility for the attack but Turkish authorities blamed Kurdistan Workers' Party (PKK) militants. "Based on recent movements and insecurity in Turkey's east, the foreign ministry advises our citizens travelling to Turkey to avoid land routes until further notice and to use air routes," a foreign ministry travel advisory read.

The Iranian bus was hit on Friday morning in Dogubayazit district of the eastern Agri province. The driver was fatally shot in the head. The passengers were unhurt, Hossein Ghassemi, Iran's consul in Erzerum, Turkey told Iranian state television Saturday. Turkish President Recep Tayyip Erdogan ordered in late July air bombing of Islamic State group jihadists in Syria and also PKK fighters based in northern Iraq.

Turkey's air strikes followed a series of attacks inside its territory including a devastating suicide bombing blamed on IS. Turkey has since suffered multiple attacks. On July 31, Iran suspended train services to Ankara after two bomb blasts on the railway in eastern Turkey. And a bomb exploded last month on the Iran-Turkey gas pipeline, leading to a days-long delay in the transport of gas in Agri province. The PKK has said a truce with Ankara that had largely held since 2013, while peace talks were ongoing, has been rendered meaningless by Turkey's air strikes in northern Iraq.

Moreover, Several thousand people marched Saturday through the western German city of Cologne in protest at continued Turkish air strikes against Kurdish separatists. Some 5,000 people demonstrators gathered in the centre of the city, police said, with organisers hoping that more of the region's substantial Kurdish and Turkish populations more would join them later.

Another much smaller protest was held in Brussels. The rallies were called by an alliance of pro-Kurdish groups to say "No to the war", after the two weeks of Turkish air raids on Kurdish separatist targets, mostly in northern Iraq.

The outlawed Kurdistan Workers' Party (PKK) has struck back killing at least 20 members of the Turkish security forces. March organiser Zubeyir Aydar of the Kurdistan Communities Union (KCK) -- which Ankara claims is the PKK's political wing -- accused Turkish President Recep Tayyip Erdogan of breaking a truce with separatists which had held since 2013. German Foreign Frank-Walter Steinmeier meanwhile told the Rheinische Post daily Saturday that it "would be regrettable that Turkey's national peace process should go to the wall because of regional conflicts in the Middle East." Turkey's main Kurdish political leader Selahattin Demirtas accused Ankara on Thursday of using air strikes against the Islamic State group as a cover to hit the PKK and to weaken his Kurdish Peoples' Democratic Party's (HDP), which made major gains in parliamentary elections in June.