Turkey hit Kurdish and Syrian regime targets on Saturday as Ankara mulled a ground assault with Saudi troops, further complicating efforts to end the war just days after the US and Russia agreed on a "cessation of hostilities" in Syria within a week.
State-run news agency Anatolia said the armed forces shelled Syrian Kurdish Democratic Union Party (PYD) targets around the town of Azaz, and also responded to regime fire on a Turkish military guard post in Turkey's southern Hatay region.
There were no further details on the nature of the Turkish strikes, which triggered alarm in Washington, but they probably involved artillery fire from tanks.
The Syrian Observatory for Human Rights said Minnigh airbase, recently taken by the Kurdish People's Protection Units (YPG) militia from Islamist rebels, was hit in the Turkish shelling.
Ankara considers the PYD and its YPG militia to be branches of the Kurdistan Workers Party (PKK) which has waged a decades-long insurgency against the Turkish state.
Saturday's shelling came shortly after Turkish Prime Minister Ahmet Davutoglu said Ankara would, if necessary, take military action against the PYD.
"We can if necessary take the same measures in Syria as we took in Iraq and Qandil," he said in a televised speech, referring to Turkey's bombing campaign last year against PKK targets in their Qandil mountain stronghold in northern Iraq.
Also in the Aleppo region, which has taken centre stage in the conflict, US-backed Syrian Democratic Forces, an alliance of Kurdish and Arab fighters, launched a two-pronged attack on Tal Rifaat, one of the remaining rebel bastions north of Aleppo city, the Observatory said.
It said Tal Rifaat also came under attack in at least 20 Russian air strikes on Saturday.
The US State Department said it was concerned about the situation north of Aleppo, was working to "de-escalate tensions on all sides" and urged Turkey to halt its strikes.
"We have urged Syrian Kurdish and other forces affiliated with the YPG not to take advantage of a confused situation by seizing new territory," US State Department spokesman John Kirby said in a statement.
"We have also seen reports of artillery fire from the Turkish side of the border and urged Turkey to cease such fires."
'Saudi ready to take part'
With the conflict directly drawing in more international players, Turkey's Foreign Minister Mevlut Cavusoglu, quoted in Turkish newspapers, said Riyadh and Ankara were coordinating plans to intervene in Syria, where Russia has been backing a successful regime offensive against rebels.
"If there is a strategy (against Daesh), then Turkey and Saudi Arabia could enter into a ground operation," he said.
Cavusoglu said Saudi Arabia is also sending planes to the Turkish base of Incirlik, a key hub for US-led coalition operations against IS already used by Britain, France and the United States for cross-border air raids.
Turkish media later quoted military sources as saying between eight and 10 Saudi jets would be deployed in Incirlik within the coming weeks, with four F-16 fighters to arrive in a first wave.
Asked if Saudi Arabia could send troops to the Turkish border to enter Syria, Cavusoglu said: "This is something that could be desired but there is no plan. Saudi Arabia is sending planes and they said 'If the necessary time comes for a ground operation then we could send soldiers'."
Saudi Foreign Minister Adel Al Jubeir, meanwhile, said in a German newspaper interview: "There is discussion on whether ground troops are needed against IS.
"If a decision is taken to send in special units against IS, Saudi Arabia is ready to take part."