The United States has halted a secretive military intelligence cooperation program with Turkey that for years helped Ankara target PKK terrorists, four U.S. officials told Reuters.

The U.S. decision to indefinitely suspend the program, which has not been previously reported, was made in response to Turkey's cross-border military operation into Syria in October, officials added.

The U.S. officials, who requested anonymity due to the sensitivity of the matter, said the U.S. late last year stopped flying the intelligence collection missions that targeted the PKK, which both the U.S. and Turkey classify as terrorists.

The U.S. military had carried out the missions using unarmed drones, which one official said were flown out of Turkey's Incirlik Air Base, where the U.S. military has a significant presence.

The U.S. drone flights that took place within the program, in place since 2007, often zeroed in on mountainous territory in northern Iraq near the Turkish border, another official said.

A Pentagon spokeswoman did not directly comment on any specific programs but noted that the U.S. has designated the PKK a terrorist organization since 1997.

"We have supported Turkey in their fight against the PKK in many ways for decades. As a matter of policy, we do not provide details on operational matters," the spokeswoman said when asked about a halt in assistance.

A State Department spokesperson said the U.S. does not comment on intelligence matters.

Officials from the Turkish defense ministry did not respond to a request for comment, but a Turkish official confirmed the program was stopped.

"This makes the anti-PKK campaign more difficult and more costly for Turkey," one of the four U.S. officials, speaking on the condition of anonymity, claimed, even though Turkey has proven self-sufficient in drone technology in recent years.

It also adds to a laundry list of grievances between the U.S. and Turkey, including Ankara's purchase of Russian air defenses and broader splits over U.S.' support to terrorist groups in Syria, particularly PKK affiliate YPG/PYD, despite what appears to be a strong relationship between President Recep Tayyip Erdoğan and his U.S. counterpart Donald Trump.

"In recent years, Turkey has not been struggling to obtain the information it needs through drones it produces itself," the Turkish official said. "However, as an ally, the steps taken on this issue do not contribute to ties between the two countries."

SPLITS OVER SYRIA

Trump, long a skeptic of U.S. military involvement in Syria, has been blamed by Democrats and even some Republicans for abandoning the U.S.-backed YPG terrorists to the Turkish onslaught, and in so doing, unraveling U.S. policy.

Turkey views the YPG as a terrorist organization, indistinguishable from the PKK.

The PKK took up arms against the Turkish state in 1984, waging an insurgency for autonomy in Turkey's largely Kurdish southeast. Since then, more than 40,000 people have been killed in the conflict. Kurds, as an ethnic group, form about 20% of Turkey's population.

Turkey's military has often struck targets in Iraq's Kurdish region near the PKK's stronghold in the Qandil mountains and has also carried out cross-border operations into northern Iraq targeting the militant group.

Since the inception of the secretive U.S. intelligence cooperation program, Ankara has invested hundreds of millions of dollars to advance its own defense capabilities and significantly reduced its dependence on U.S. and Israeli drones which it frequently used since the late 1990s.

Turkey's privately-owned Baykar Defence has been working on developing Turkey's first drone fleet since the 2000s.

Within a decade and a half, it has developed armed and unarmed drones and begun selling them to the Turkish army as well as to Ukraine and Qatar.

As of July 2019, a total of 86 Bayraktar drones are in service with Turkey's security forces and some of those have been regularly used during Ankara's three Syria operations in 2016, in 2018 and again last October.

Arda Mevlütoğlu, a Turkey-based defense analyst said the recent advance has equipped Ankara with greater flexibility and freedom in its operational capabilities.

"Turkey's dependence on her allies, mainly to the U.S., significantly decreased, if not completely ended in real-time, high-quality intelligence gathering and surgical strike type operations," Mevlütoğlu said.