CUKURCA (Reuters) - Turkey and Iran pledged Friday to cooperate in the fight against Kurdish militants, as thousands of Turkish troops pressed ahead with an air and ground offensive for a third day following an attack that killed 24 Turkish soldiers. The counter-insurgency operation against separatist fighters from the Kurdistan Workers' Party (PKK) was concentrated on both sides of Turkey's border with northern Iraq. Hundreds of Turkish soldiers were hunting PKK fighters around the Zab river areas a few kilometers inside Iraqi territory, security officials said. Turkey's reaction to one of the most deadly attacks on its security forces in a conflict that began three decades ago has ignited speculation that Turkey could move to a full blown incursion to clear out PKK camps deeper inside northern Iraq. The prospect would heighten risks in an already unstable region, with US troops due to withdraw from Iraq this year, and neighboring Syria in the grip of a brutal repression against pro-democracy protesters. Turkey, Iraq, Syria and Iran all have large ethnic Kurdish minorities, but the separatist struggle is fiercest in Nato member Turkey, where more than 40,000 people have been killed in the violence.The neighborhood is rife with suspicions over who could be supporting the Kurdish militants.