Turkish jets bombed the Kurdish-controlled city of Afrin in northern Syria on Saturday, as President Recep Tayyip Erdogan promised to expand Turkey's military border operations against a Kurdish group that has been the U.S.'s key Syria ally in the war on the group Islamic State in Iraq and Syria (ISIS).

The warplanes attacked Afrin shortly before sunset, Turkish media reported. A Kurdish official confirmed the strikes saying they were the first by the Turkish military on the city, in what it has named operation "Olive Branch."

The attack comes on the heels of a week of sharp threats by the Turkish government, promising to clear the Kurdish People's Protection Units, or YPG, from Afrin and its surrounding countryside, also called Afrin.

Turkey says the YPG — a group it considers a terrorist organization — is an extension of an outlawed Kurdish rebel group that it is fighting inside its own borders. It has recruited to its cause thousands of disaffected Syrian opposition fighters, who view the YPG as a counter-revolutionary force in Syria's multi-sided civil war.

Smoke billows on the Syrian side of the border near Turkey on Saturday after Turkish jets struck targets that Ankara said belonged to the People's Protection Units, a mainly Kurdish militia in Syria. (Bulent Kilic/AFP/Getty Images)

Associated Press journalists at the Turkish border saw a convoy of buses, believed to be carrying Turkish-backed Syrian opposition fighters, travelling along the border across from Afrin. The convoy included at least four trucks carrying pickup trucks mounted with machine guns. Video from Turkey this week showed the military moving tanks to the border.

The YPG is the driving force behind a coalition of north Syrian forces allied with the U.S. to battle the Islamic State group. With U.S. support, including close to 2,000 embedded forces, the coalition now controls close to a quarter of Syrian territory, concentrated mostly in the north east.

Attack coincides with U.S. announcement

Turkish leaders were infuriated by an announcement by the U.S. military one week ago that it was going to create a 30,000-strong border force with the Kurdish fighters to secure northern Syria. Days later, Secretary of State Rex Tillerson announced that the U.S. would maintain a military presence with the Kurds for the foreseeable future.

Speaking in the city of Kutahya in western Turkey, Erdogan announced an expansion to Turkish operations in Syria, promising to move on the Kurdish-controlled town of Manbij and its surrounding countryside after completing operations in Afrin, to force out the Kurdish militia from all positions west of the Euphrates river.

Turkish Prime Minister Binali Yildirim said the strikes on Afrin marked the start of a campaign to "eliminate the PYD and PKK and Daesh elements in Afrin," referring to the Kurdish Democratic Union Party and the Kurdistan Worker's Party respectively, and using an Arabic acronym for the Islamic State group. The PYD, PKK, and YPG all look to the Kurdish Marxist-nationalist leader Abdullah Ocalan as their guide. Ocalan is imprisoned by Turkey for waging a separatist movement in the eastern part of the country.

The airstrikes were preceded by waves of artillery strikes on the Afrin region.

Damascus threatens to shoot down jets

Any ground operation would entail considerable military and political risk for Ankara. Russia keeps military observers in Afrin and has lately firmed up its ties with the YPG. Syria's government in Damascus says it will shoot down any Turkish jets on raids in the country. The YPG is estimated to have between 8,000 and 10,000 fighters in Afrin.

Turkey could also face blowback from the Kurdish insurgency within its own borders.

A ground offensive or continued shelling would exacerbate the poor humanitarian situation in Afrin, which is now home to at least 800,000 civilians, including many who arrived fleeing the fighting in other parts of Syria.

Russia has observers in Afrin

Turkey's military and intelligence chiefs travelled to Moscow on Thursday to discuss Turkey's planned operation. Russia has not removed its observers from Afrin.

Also on Saturday, Syrian government forces and supporting militias retook a key air base in northwest Syria lost to rebels in 2015.

Syria's state broadcaster said Syrian troops fought their way into the strategic Abu Zuhour air base on Saturday, in Idlib province.

It was a coup for the government and allied militias who advanced swiftly to take the base in what was considered a stronghold for rebels and al-Qaeda insurgents.

But the advance, which began in earnest in late December, has displaced more than 200,000 civilians, according to the UN, exacerbating an already dire humanitarian situation in the north of the country.

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