Turkish artillery on Saturday shelled areas of Aleppo province in northern Syria held by Kurdish forces, a monitor said, as Ankara said it could launch a ground assault alongside Saudi troops.
The Syrian Observatory for Human Rights said Turkish artillery struck areas of Aleppo, including Minnigh airbase recently taken by the Kurdish People's Protection Units (YPG) militia from Islamist rebels.
Ankara considers the Syrian Kurdish Democratic Union Party (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.
The 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. The premier was referring to Turkey's bombing campaign last year against PKK targets in their Qandil mountain stronghold in northern Iraq.
A YPG source told AFP that the Turkish shelling targeted the strategic Minnigh airbase, which Kurdish forces retook late on Wednesday.
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 the Islamic State jihadist group), 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 ISIS already used by Britain, France and the United States for cross-border air raids.
"They (Saudi officials) came, did a reconnaissance of the base. At the moment it is not clear how many planes will come," Cavusoglu said.
Turkish media later quoted military sources as saying between eight and 10 Saudi jets would be deployed in Incirlik within the coming weeks, with 4 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 ready to take part"
Saudi Foreign Minister Adel al-Jubeir, for his part, said in a German newspaper interview: "There is discussion on whether ground troops are needed against ISIS. If a decision is taken to send in special units against ISIS, Saudi Arabia is ready to take part."
In an interview with AFP released on Friday, Syrian President Bashar al-Assad said he "doesn't rule out" that Turkey and Saudi Arabia would intervene militarily in Syria, but said that his armed forces "will certainly confront it."
Saudi Arabia had already said earlier this month that it was ready to join any ground operation against ISIS.
Turkey's relations with fellow mainly Sunni Muslim power Saudi Arabia have warmed considerably in recent months. Ties had been damaged by Saudi's role in the 2013 ouster of Egyptian president Mohammed Morsi, a close ally of Ankara and a member of the Muslim Brotherhood.
Saudi Arabia and Turkey are both staunch supporters of rebels seeking to oust Assad and see his overthrow as essential for ending Syria's five-year civil war that has cost more than 260,00 lives. Assad is backed by Shi'ite Iran, a key rival of the Saudis.
They fear the West is losing its appetite to overthrow him on the assumption he is "the lesser of two evils" compared to the ISIS jihadists.
"New Cold War"
Both are outraged by the Russian military intervention in Syria, which analysts believe has given Assad a new lease of life and has also deeply alarmed the West.
Russian Prime Minister Dmitry Medvedev said Saturday that strains between Russia and the West over the Syrian and Ukraine conflicts had plunged the world into a "new Cold War."
"Almost every day we are accused of making new horrible threats either against NATO as a whole, against Europe or against the US or other countries," Medvedev said.
He also warned last Thursday that the entry of Arab armies into Syria could spark a "new world war."
US Secretary of State John Kerry complained that the vast majority of Russia's attacks in Syria were against "legitimate opposition groups" rather than ISIS jihadists. "We think it is critical that Russia's targeting change," he said.
Russia also dispatched a new patrol ship armed with cruise missiles to the Mediterranean, with reports saying it was bound for Syria.
An ambush by rebels on pro-regime forces near Damascus this week killed 76 fighters, the Syrian Observatory said on Saturday.
World powers on Friday announced an ambitious plan to stop fighting in Syria within a week, but doubts have emerged over its viability, especially because it did not include ISIS or Al Qaeda's local branch, which is fighting alongside other rebel groups in several areas.
AFP contributed to this report.