Turkey said Wednesday it was more determined than ever to secure its Syrian border from attacks by Kurdish forces, threatening a ground operation "at the most convenient time".
Ankara launched a campaign of air strikes across parts of Iraq and Syria on Sunday as part of Operation Claw-Sword, following a bombing in Istanbul on November 13 that killed six people.
"Our operations with planes, cannons and drones are only the beginning. Our determination to secure all our southern border... with a safe zone that is stronger today than ever before," President Recep Tayyip Erdogan told lawmakers in parliament.
"While we press ahead with air raids uninterrupted, we will crack down on terrorists also by land at the most convenient time for us," he told lawmakers from his AKP party.
Turkey blamed the Istanbul attack on the Kurdistan Workers' Party (PKK), which is blacklisted as a terror group by the European Union and the United States.
The PKK, which has waged an insurgency against the Turkish state since 1984, denied any role in the bombing -- the deadliest in five years after a spate of attacks in Turkey between 2015 and 2017.
Turkey has intensified its air raids and artillery fire on PKK and Kurdish People's Protection Units (YPG) targets in northern Iraq and Syria.
Ankara considers the YPG a terror group linked to the PKK. But the YPG -- the dominant force within the US-backed Syrian Democratic Forces (SDF) is seen as crucial in the fight against the Islamic State (IS) extremist group.
- 'Can't keep their word' -
The YPG has also denied a role in this month's Istanbul attack.
A Kurdish military official told AFP that a Turkish drone strike hit a Kurdish position inside a Russian base in northeastern Syria on Wednesday killing one Kurdish fighter and wounding three others.
The casualties were all fighters of the Kurdish-led SDF, the SDF officer said.
A deadly Turkish drone strike against a base used jointly by Kurdish and American forces in northeast Syria did put US troops in danger after all, Central Command said Wednesday.
The drone strike, against a base north of Hasakeh on Tuesday, killed two fighters belonging to the SDF.
"We have received additional information that there was a risk to US troops and personnel," Centcom said in a new statement to AFP on Wednesday, without elaborating.
Erdogan accused Western states backing Kurdish forces in Syria of failing to deliver on their promise of no attacks on Turkish soil, pointing to the Istanbul bombing.
"If they can't keep their word," he said, "then we have the right to take care of ourselves."
Russia and the United States have called for de-escalation.
Late Tuesday, YPG spokesman Nuri Mahmoud deplored "the timid and weak statements by the guarantor countries and International Coalition" which relied on the group to counter IS jihadists.
Erdogan has repeatedly called for a 30-kilometre (19-mile) "safe zone" to protect Turkey against cross-border attacks from Syrian territory.
On Wednesday, he vowed Turkey would complete the zone "step by step" starting from the towns of Tal Rifaat, Manbij and Ayn al-Arab, also known as Kobane in Kurdish, which are all located in northern Syria.
Kobane, a Kurdish-majority town near the Turkish border, was captured by IS in late 2014 before Syrian Kurdish forces drove them out early the following year.
A Turkish official told AFP the air campaign was "likely to continue for a while" and when the conditions were ripe, Turkish troops inside Syria would start the land incursion, referring to the presence of Turkish military units on the Syrian side of the border for three years.
Ankara has launched waves of attacks on Syria since 2016 targeting Kurdish positions as well as IS jihadists.
A joint statement after a tripartite meeting between Russia, Turkey and Iran on Syria said the parties "emphasised their strong support to Syria's territorial integrity."
They also stressed "their determination to fight against the separatist agendas that pose threat to the national security of neighboring countries," after a meeting in the Kazakh capital of Astana.
- Gas plant -
Turkish Defence Minister Hulusi Akar said the military had hit 471 Kurdish targets across Iraq and Syria during its latest offensive, adding "254 terrorists were neutralised."
A British-based monitoring group and Kurdish authorities have reported Turkish drone strikes on many locations in Hassakeh province including a domestic gas plant and an oil pumping station.
Turkish artillery fire also hit near the Jerkin prison in Qamishli, which holds Islamic State group detainees, according to both sources.
Akar insisted Wednesday that Turkey's military campaign targeted only "terrorists" and their structures.
"We have no problem with any ethnic or religious group, with our Kurdish or Arab brothers."
Turkish border towns have come under rocket fire from Syria in recent days. At least three people, including a child, were killed in a Turkish border town on Monday.