ANKARA (AFP) - Turkey on Saturday vowed a strong response after 15 Turkish soldiers and 23 Kurdish rebels were killed in heavy clashes sparked by a rebel attack on a military post near the Iraqi border. The fighting, the bloodiest this year, is likely to ratchet up pressure on Ankara to hit back at the militants who use rear bases in neighbouring northern Iraq to strike Turkish targets in its 24-year separatist campaign. President Abdullah Gul cancelled a planned visit to France to deal with what he described as a "treacherous" attack and said Ankara was determined to crush the rebels. "Let me underline once again that we will continue the struggle (against the PKK), whatever the cost," Gul said in a video message just before he held a meeting with the army chief. "We are investigating how this treacherous attack took place, who facilitated it. These will be followed up on and everyone will be held to account," he added. Gul was to have delivered a speech to an international conference in France on Monday, his office said. The foreign ministry said it had contacted Iraqi authorities and urged them to take action to tackle PKK rebels operating out of the Kurdish-run autonomous enclave in the north of Iraq. "We expect the government of our neighbour Iraq to fulfil its responsibilities," it said in a statement.Turkish Prime Minister Recep Tayyip Erdogan immediately cancelled a two-day visit to Mongolia and left Turkmenistan, where he was on an official trip, for Turkey for emergency talks with the army leadership and several cabinet ministers. The European Union, which Turkey is seeking to join, strongly condemned the latest violence by the rebels and reiterated its solidarity with Turkey against the PKK. The Turkish army said that PKK rebels attacked a border post in Semdinli town, Hakkari province, on Friday afternoon under cover of heavy weapons' fire from northern Iraq, killing 15 soldiers. Turkish forces responded with artillery fire and attack helicopters pounded rebel positions while additional forces were dispatched to the area. Turkish fighter jets and artillery units also struck at a group of rebels in the north of Iraq, about 10 kilometres from the station, which was attacked. "Twenty-three terrorists were neutralised in the clashes. It is not yet clear how many terrorists were killed by artillery fire and in the strikes by the air force," General Metin Gurak, the head of the general staff's press department told the Anatolia news agency. He added that two soldiers remained unaccounted for after the fighting which lasted late into Friday night. Twenty soldiers were also wounded in the clashes. The PKK also confirmed the attack and said it was a response to continuing Turkish military operations despite a unilateral ceasefire the rebels announced to mark Eid Tuesday and Thursday.