Turkey intensified its shelling of Kurdish fighters in neighboring Syria on Friday as it ramped up threats to launch a full military offensive across the border, a move that would stoke tensions with the United States.
Senior Turkish officials have been warning for days of an imminent strike into the Kurdish-held region of Afrin in northern Syria, after the US announced it was training a 30,000-strong border force in Syria made up predominantly of Kurdish fighters.
The Turkish Defense Minister said on Friday that the operation was "de facto" underway.
Turkish President Recep Tayyip Erdogan accused the US -- its most powerful NATO ally -- of creating an "army of terror" along its southern border.
Ankara, which has long fought Kurdish unrest in southeastern Turkey, is determined to prevent the establishment of a Kurdish state across the border in Syria. It has used military force in the past against Kurds in northern Syria, who have seized control of swathes of territory in the fight against ISIS.
But launching a new operation solely targeting Kurdish fighters could open up a new frontier in the Syrian civil war, which has lasted for nearly seven years. It would also sharpen tensions between Turkey and the US, which supports and now openly arms Kurdish militias fighting ISIS.
A US official told CNN Friday that Washington was seriously concerned about a new operation and that the possibility of a full Turkish military incursion into Syria is the "major topic we are watching this weekend."
More than 50 Turkish vehicles, including artillery, tanks, rocket launchers and heavy equipment transporters have been observed on the Turkish-Syrian border, according to another US defense official.
In a sign of mounting tensions in the region, two other US defense officials told CNN Friday that Turkish backed rebels near Manbij, Syria are regularly firing on US military patrols and that US forces occasionally return fire. Another US military official said US troops advising local allies in the Manbij area came under fire from these Turkish backed rebels "within the last week."
State Department spokeswoman Heather Nauert said the US called on Turkey to refrain from launching the operation. "The focus needs to be on ISIS. So we don't want them to engage in violence, but we want them to keep focused on ISIS," she told reporters Thursday.
Turkish Defense Minister Nurettin Canikli told A Haber TV on Friday that the shelling showed that the "operation has de facto begun."
"What we can say is this: This operation will be carried out. The area will be cleansed of terrorist organizations. This could be today, it could be tomorrow, it could be tonight. The Afrin center operation may take a while but the terrorist organization will disassemble quickly," he said.
Turkish media has reported the movement of a large number of troops and tanks to the Syrian border in recent days.
Syria 'ready to destroy' Turkish planes
The Turkish news agency Anadolu reported that Russian forces were leaving the area and that the Turkish army's chief of staff, Hulusi Akar, met with senior Russian defense officials in Moscow on Thursday to seek Russia's support.
Russia is Syria's most powerful ally. Turkey is working with both Russia and Iran to broker a peace deal to end the war.
Turkish Foreign Minister Mevlut Cavusoglu said that officials were consulting Russia and Iran about the use of airspace for the operation, CNN Turk reported.
He called on Russia to support the operation and said Turkey would work with the country "to prevent any mishaps" that could affect Russian forces in the area.
The Russian Defense Ministry has not yet responded to a request for comment from CNN.
Turkish state-run media has suggested that the new operation would involve airstrikes from warplanes and drones, and that Turkish-trained militia from the Free Syrian Army alliance would be first on the ground in any land offensives.
Syrian Deputy Foreign Minister Faisal Mekdad warned the operation would be considered a "hostile act" against the country and that Syrian army was "ready to destroy" Turkish planes in Syrian skies.
"Our worry is deep and the beating of war drums by the Turkish leadership is displeasing," Mekdad said.