ANKARA (Reuters) – Fifteen Turkish soldiers were killed in clashes with Kurdish separatist PKK rebels in southeast Turkey on Friday, Turkey’s General Staff said, in one of the deadliest attacks on the military this year.

At least 23 members of the Kurdistan Workers’ Party (PKK) were also killed after rebels armed with “heavy weapons” attacked a military outpost in the Semdinli region bordering Iraq and Iran, the military said.

The attack is likely to put pressure on Prime Minister Tayyip Erdogan to strike back at the PKK.

Turkey has attacked PKK bases in mountainous northern Iraq several times over the past 12 months, but has confined itself to shelling and air strikes since a brief land offensive in February.

The General Staff said two Turkish soldiers were missing and that an operation was under way to rescue them. It said the rebels had received back-up from PKK bases in Iraq.

Erdogan was on an official visit to Turkmenistan on Saturday but said he was returning to Turkey for an emergency security meeting.

“The struggle against terrorism will continue with the same determination as before,” he told an audience in Ashgabat.

Parliament is this month likely to approve a government request to extend a mandate to launch military operations against the PKK in Iraq as needed. The current mandate expires on October 17.

Turkey blames the PKK, considered a terrorist organization by the United States and the European Union, for the deaths of more than 40,000 people since it launched its campaign for an ethnic Kurdish homeland in southeast Turkey in 1984.

Washington is sharing vital intelligence with Turkey on PKK movements in Iraq. The United States and the European Union have expressed concerns that prolonged Turkish military operations inside Iraq could further destabilize Iraq and the wider region.

Friday’s attack is the first serious challenge for Turkey’s new military chief, General Ilker Basbug, who took over NATO’s second-largest army in August.

Basbug has said that, while military operations against the PKK will continue, socio-economic measures are needed to bring peace to the impoverished southeast.

Erdogan has announced plans to invest up to $12 billion in southeast Turkey over five years in an effort to improve living standards and drain support for the PKK.

The ruling AK Party hopes to wrest control of key cities in the southeast such as Diyarbakir from a pro-Kurdish political party in municipal elections next March.

Turkey is home to around 12 million Kurds, a sixth of the total population.