Turkey's state media has said the strikes killed around 200 YPG fighters, although the death roll is disputed. Turkey has voiced its displeasure at US support of Kurdish groups fighting the so-called "Islamic State."
Turkish jets and artillery forces launched more than 20 airstrikes on a US-backed Kurdish militia group in northern Syria on Thursday, highlighting the widening gap between Washington and Ankara's military agendas for the region.
According to Turkish state media, the airstrike killed between 160 and 200 combatants from the People's Protection Unit (YPG), the strongest force in the Kurdish-led Syrian Democratic Forces (SDF). However, the death toll has been widely disputed. The British-based Syrian Observatory for Human Rights said at least 11 fighters were killed, while the early reports from the YPG itself suggest no more than 10 were killed.
The warplanes targeted three villages occupied by YPG personnel in the Afrin region to the northeast of Aleppo. Kurdish fighters had recently captured the villages from the so-called "Islamic State" (IS).
Footage released by the pro-Kurdish Anha news agency reportedly shows smokes billowing out from an SDF camp following the air strike. They tweeted the footage, saying "Turkish warplanes hit civilians."
The strikes, the strongest against YPG forces since Turkey launched its incursion into Syria in August, came mere hours after President Recep Tayyip Erdogan said that Turkey could act alone in rooting out its enemies and that it "would not wait for terrorist organization to come and attack."
Senior Kurdish official Ilham Ahmed said on Thursday that Turkey's attacks on US-backed Kurdish forces posed a threat to the United States' fight against terrorism in the region. She accused Ankara of taking advantage of the US' preoccupation with the presidential election by imposing its own plans for the region. She urged Washington to intercede immediately.
US Defense Secretary Ash Carter is expected to visit Ankara on Friday.
The Turkish military said the attacks were retaliatory after five shells fired from the YPG-controlled Afrin region hit Turkey's Hatay province, striking empty land.
Turkey also announced that 21 militants from the PKK, a Kurdish separatist organization in Turkey and Iraq, were killed in military operations in the southeastern Turkish province of Hakkari.
Turkey is a significant backer of the insurgency against Syrian President Bashar al-Assad, supporting and arming the Free Syrian Army. However, Ankara admits that its intervention is also aimed at preventing the SDF from gaining more ground in the region. Both Turkish and Kurdish forces are racing to capture key territories from IS.
Thursday's bombardment comes as Kurdish forces made significant territorial gains in the fight against IS, including in the Aleppo province. The United States has backed such Kurdish fighters, albeit to Ankara's displeasure. It views the YPG as an extension of Kurdish separatist groups that have waged a three-decade insurgency in the southeast of Turkey.
Turkish officials have grown increasingly wary that Kurdish forces will use the captured territories and connect its three de facto autonomous cantons, creating a Kurdish-run enclave in northern Syria and stoking separatist ambitions in Turkey.
Numan Kurtulmus, Turkey's deputy prime minister, said his country was "displeased" with the US' support of Syrian Kurdish fighters. He told reporters on Thursday that "whoever comes next to the US presidency" must understand the importance of maintaining ties with "a key regional country like Turkey and not an armed terrorist organization with a few thousand militants."
Commander Mahmoud Barkhadan of the People's Protection Units (YPG) accused Turkey of aiding IS insurgents by converting the fight against the Islamist extremists into a Turkish-Kurdish conflict. "We are fighting Daesh, why are they striking at us?" he told the Associated Press news agency. He said that Kurdish forces had not retreated from the newly captured territories, but that the Turkish airstrikes had allowed IS militants to launch a counteroffensive.
Fighting between Kurdish forces and IS continued into Thursday.