Kurdish rebels killed at least 24 soldiers Wednesday in southeast Turkey in one of the deadliest days in the 27-year battle against the separatists, sparking a military offensive on land and air.
Ankara retaliated with cross-border air strikes on rebel bases and sent troops into neighbouring Iraq as Prime Minister Recep Tayyip Erdogan cancelled a foreign trip and called an emergency meeting of top security officials.
The attacks by Kurdistan Workers' Party (PKK) rebels occurred in eight locations in Cukurca and Yuksekova in Hakkari province near the Iraqi border in the early hours of Wednesday, local security sources said.
"According to the latest confirmed information from Cukurca, the conflict region, 24 of our soldiers fell martyr and 18 of them were wounded," Erdogan said in televised remarks.
"Currently, large-scale operations including hot pursuit (of rebels in Iraq) go on in accordance with international law," said Erdogan, who cancelled an official visit to Kazakhstan.
The toll -- earlier put at 26 by health officials in Turkey's southeast -- is the heaviest for the army since 1993, when the PKK killed 33 unarmed soldiers in Bingol province, in southeast Turkey.
The rebel hits drew sweeping condemnation.
US President Barack Obama denounced the "outrageous terrorist attack" and vowed to "continue our strong cooperation with the Turkish government as it works to defeat the terrorist threat from the PKK."
UN Secretary General Ban Ki-moon called on Iraq and Turkey to collaborate on a solution to end what he called "unacceptable" cross-border attacks.
Ban's spokesman Martin Nesirky said the sovereignty of both Turkey and Iraq must be respected.
"The secretary general expresses his concern over the attacks in the Hakkari province of Turkey, reportedly by Kurdish elements operating from Iraqi territory, which have left many Turkish soldiers dead," he said in a written statement.
"It is clearly unacceptable that Iraq's territory is being used to mount cross-border attacks against neighboring countries," Nesirky said.
"The secretary general urges Iraq and Turkey to engage in constructive dialogue in order to find a peaceful solution to this challenge."
German Foreign Minister Guido Westerwelle urged all sides to refrain from further violence and said he was "alarmed and shaken" by the high number of victims.
The Kurdish separatist group threatened Turkey with worse if the army follows through with a ground incursion into Iraq's autonomous north.
"The Turkish army will take a bigger hit if they try to carry out any military operation outside of the Turkish border," rebel spokesman Ahmed Denis told AFP.
PKK said in a statement on Wednesday that it launched the attacks in Hakkari in retaliation for earlier Turkish air strikes against its bases in northern Iraq and the arrest of hundreds of Kurdish politicians around Turkey, Kurdish news agency Firatnews said.
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Five rebels were killed in the latest attacks in Hakkari, PKK said.
Several hundred Turkish soldiers have crossed into northern Iraq to hunt down PKK rebels, Firatnews said.
"Turkish soldiers from two separate points in Cukurca town crossed into south Kurdistan to follow (rebels)," said the agency, which is known as a mouthpiece of the PKK.
Another PKK spokesman, Dozdar Hammo, told AFP that Turkish troops "are attempting to cross the Iraqi border at Jeli," in the southeastern Hakkari region.
Turkish army commandos had been flown in by helicopter to the Iraqi side of the border, local security sources said, as Turkish air force planes bombed Kurdish rebel bases in Iraq.
The air raids targeted Qandil region, the main rear base of the PKK, and Zap region, they added.
Turkey has bombed rebel bases in Iraq several times in recent months, killing nearly 100 rebels according to the army general staff.
Parliament in Ankara earlier this month renewed the government's mandate to conduct ground incursions into Iraq.
"Both our friends and enemies should understand well that we will never give in to any attacks from inside or outside, never step back and sacrifice even one scrap of the land," Erdogan said.
He was speaking after gathering his top security officials, including the interior and defence ministers and military planners, in his office in Ankara.
President Abdullah Gul said on television that Turkey "will not be shaken by terror ... We will do whatever we can do to finish this."
NATO and the Council of Europe also condemned the attacks.
"On behalf of NATO, I condemn in the strongest possible terms the recent attacks in southeastern Turkey, which killed and injured several Turkish soldiers, policemen and civilians, including a child," its secretary general Anders Fogh Rasmussen said.
"Terrorists will never achieve their ends through violence and bloodshed. They do not defend ideas, they seek only to propagate hatred and barbarity. Their acts are a crime against us all," the Council of Europe added.
Authorities in Iraq's autonomous Kurdistan region meanwhile branded the attacks as a "criminal act."
Clashes between the PKK and the army have escalated since the summer.
Five police and four civilians were killed in a landmine explosion in the southeast on Tuesday.
The PKK, listed as a terrorist organisation by Turkey and much of the international community, took up arms in Kurdish-majority southeastern Turkey in 1984, sparking a conflict that has claimed about 45,000 lives.