Turkey on Monday accused the United States of supporting "terrorists" and summoned its ambassador after Washington declined to immediately back Ankara's claim of Kurdish militants executing 13 of its nationals in Iraq.

The Turkish interior ministry also announced the detention of hundreds of people it accuses of links to the outlawed Kurdistan Workers' Party (PKK) rebels that allegedly shot dead the 13 hostages in an Iraqi cave.

The PKK blamed Turkish airstrikes for the deaths in a diplomatically-charged episode that delivers an immediate challenge to the new US administration of President Joe Biden in the Middle East.

Ankara on Sunday said the hostages — most of them soldiers and police — were executed as Turkish forces advanced in a rescue operation launched in the mountains of northern Iraq last week.

The Turkish anger was directed at a US State Department statement on Sunday saying Washington "deplores the death of Turkish citizens” but awaits further confirmation of Ankara’s version of events.

"If reports of the death of Turkish civilians at the hands of the PKK, a designated terrorist organisation, are confirmed, we condemn this action in the strongest possible terms,” the State Department said.

Turkish President Recep Tayyip Erdogan branded Washington’s response "a farce”.

"You said you did not support terrorists, when in fact you are on their side and behind them,” he said in televised remarks.

The Turkish foreign ministry then summon US Ambassador David Satterfield to express Ankara’s displeasure "in the strongest possible terms”.

Mass arrests 

The PKK militants have been waging an insurgency against the Turkish state since 1984 that is thought to have left tens of thousands dead.

The group is recognised as a terrorist organisation by Washington and most of Turkey’s other Western allies.

But Washington works in Syria with another Kurdish militia that Turkey views as a branch of the PKK.

A breakdown in peace talks between the PKK and Ankara in 2015 was followed by Turkish military campaigns against the militants across the region and mass arrests of pro-Kurdish politicians and officials in Turkey.

The Turkish interior ministry said on Monday that it had detained 718 people — including the heads of pro-Kurdish Peoples’ Democratic Party (HDP) — in cities and districts across the country in a series of coordinated raids.

The HDP is Turkey’s second-largest opposition party. It denies all formal links to the PKK but also questions Turkey’s account of the 13 deaths.

We "express our deepest regret and condolences to the families and loved ones of those 13 people who by no means were capable of protecting themselves against an armed attack”, the HDP said in a statement on Sunday.

‘Killing innocents’ 

The incident threatens to escalate tensions across Iraq and Syria while delivering an early test to Erdogan’s relations with Biden.

Turkey has long accused the Iraqi government of being too tolerant of the PKK.

Ankara instead wants Washington to reaffirm its support for Turkey’s anti-terror campaign.

Erdogan said Turkey’s NATO allies had to pick sides.

"If we are going to be in NATO together, you should be sincere. You should not be on the terrorists’ side,” Erdogan said.

"After this, there are two options. Either act with Turkey with no ifs or buts, without questioning, or they will be a partner to every murder and bloodshed,” he said.

"The terrorist organisation on our doorstep, on our borders, is killing innocents.”

Erdogan’s rebuke of the US contrasts with his efforts over the past few months to strike a softer tone and mend torn relations with the West.

Erdogan is still waiting for a phone call from Biden that could help set the tone for future US-Turkish ties.