On consecutive days this weekend, Turkish forces struck positions of Syrian Kurds with fire from Firtina howitzers deployed on the border, saying the army responded to incoming attacks.
This was the first time Turkey had confirmed striking positions of the Democratic Union Party (PYD) and its People's Protection Units (YPG) militia inside Syria, although last year the Kurds accused Turkey on at least one occasion.
Turkey accuses the PYD and YPG of being the Syrian branch of the Kurdistan Workers Party (PKK) which has fought an over 30-year insurgency against the Turkish state in search of greater powers and rights for the country's Kurdish minority.
Ankara fears the groups are seeking to create a Kurdish autonomous region on Turkey's doorstep in Syria and is now lumping the PYD and YPG together with the regime of President Bashar al-Assad, who Ankara wants ousted, and its Russian allies.
But while the United States regards the PKK as a terror group it does not extend the classification to the PYD and YPG, who have become important allies for Washington as virtually the sole forces able to successfully fight Islamic State (IS) jihadists on the ground in Syria.
"The disagreement between the United States and Turkey regarding PYD-YPG is evident," said Ozgur Unluhisarcikli, Ankara Office Director of the German Marshall Fund of the United States.
"The United States regards the PYD-YPG as the only local actor in Syria it can partner with against Daesh (IS). Turkey on the other hand regards PYD-YPG not only as an affiliate of the PKK but also a proxy of the Russia-Assad axis."
The US State Department rapidly urged Turkey to halt the firing while also urging the Syrian Kurds "not to take advantage of a confused situation by seizing new territory".
However Turkish officials responded that Turkey would continue to hit the Kurdish fighters in Syria for as long as they were deemed a threat to national security.
"Turkey is not a nation that is going to sit down and watch everything from the sidelines or the stands," Deputy Prime Minister Yalcin Akdogan told Kanal 7 TV.
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Prime Minister Ahmet Davutoglu late Saturday spoke by telephone to US Vice President Joe Biden, telling him the PYD was a "threat" against which "we can take all kind of measures".
"The Turkish strikes... will increase the distrust and tension between the two allies who still need to work with each other very closely on multiple issues," said Unluhisarcikli.
Ankara has been alarmed by the recent military success of the PYD and has set the Euphrates River as a "red line" that must be the limit of their westwards advance.
The problem is accentuated by the fact that Turkish forces are again waging a bitter conflict with the PKK inside Turkey after the collapse of a two-and-a-half year ceasefire in June.
In some of his most bitter criticism of the United States, President Recep Tayyip Erdogan last week asked the United States: "Are you together with us, or are you with the PYD and YPG terror groups?"
"Allies don't tell each other: 'My enemy's enemy is my friend'," he said.
Writing in the Daily Sabah newspaper, Erdogan’s spokesman Ibrahim Kalin accused the Syrian Kurds of working to expand a Kurdish autonomous region in northern Syria "under the cover of fighting Daesh (IS)."
"If it (the United States) is choosing a terror organisation over Turkey, no problem, so be it," wrote the strongly pro-Erdogan editor-in-chief of the Yeni Safak daily Ibrahim Karagul.
"But we are going to say: 'The US is waging a war on Turkey'."
Anti-Erdogan commentator Cengiz Candar said the government's classification of the PYD as terrorists was irrational, noting the group had not been blamed for a single attack inside Turkey since it was founded in 2003.
"The problem stems from Ankara's Kurdish phobia which is of a kind that cannot be overcome," he wrote in the Radikal online daily. "The priorities of Turkey and the United States in Syria are not the same."