The Syrian Kurdish People’s Protection Units (YPG/YPJ) have warned that consistent threats from Turkey, which claims to also be fighting the Islamic State in Syria, “have reached the level of a declaration of war,” threatening to derail the ISIS fight just as the YPG have surrounded its “capital,” Raqqa.

“These (Turkish) preparations have reached level [sic] of a declaration of war and could lead to the outbreak of actual clashes in the coming days,” Sipan Hemo, described as a YPG “head” by Reuters, told the outlet Wednesday. “We will not stand idly by against this potential aggression.”

Both the YPG and the Turkish military are allied to the United States. The Turkish government considers the YPG a “terrorist group,” however, linking it to the U.S.-designated terror organization the Kurdistan Workers’ Party (PKK), while the Pentagon has referred to the YPG as the most effective force on the ground in Syria against ISIS. Turkey fears that the YPG will try to establish a sovereign Kurdish nation in Rojava, or Syrian Kurdistan, on Turkey’s border.

Hemo’s warning follows days of tensions growing in Afrin, Syria, near the Turkish border. On Wednesday, the Kurdish outlet Rudaw reported that the YPG has documented a number of civilian casualties in the region and attributed them to “overnight shelling” by Turkish troops.

“The YPG announced a 40-year-old woman had been killed, along with two 13 year olds on Tuesday. Additionally four people were said to be hospitalized,” Rudaw reported. “Monday, YPG announced the deaths of three people and seven others injured in the Shehba region on the outskirts of Afrin.”

Turkish outlets say the YPG is also apparently preparing for aggression in the region. Sabah, which is sympathetic to Turkish president Recep Tayyip Erdogan, reported Tuesday that Turkish media have evidence suggesting the YPG has “reportedly moved some of its heavy machinery to the Turkish border.” Some of this heavy machinery may be of U.S. origin, as American officials announced in May they would arm the YPG for the purpose of defeating the Islamic State in Raqqa.

“The YPG has recently threatened to hit Turkey if Ankara decided to launch an operation on the YPG-held towns of Afrin and Tal Rifat in northern Syria,” Sabah claimed.

These reports follow a number of similar ones suggesting that fighting has already begun from groups like the Syrian Observatory for Human Rights (SOHR), a UK-based NGO. The Chinese state media outlet Xinhua also reported Wednesday that experts on Turkish politics have suggested “a Turkish military operation is imminent” against the YPG.

“Turkey has substantially increased the presence of its troops, tanks and artillery units around Afrin over the past two weeks, while most of the Russian troops retreated last week to areas controlled by the Syrian government, according to Turkish media reports,” Xinhua notes.

The Russian government is primarily acting in Syria as an ally of Syrian dictator Bashar al-Assad. While Erdogan once said the only reason he deployed troops to Syria was to “end the rule of the tyrant Assad,” Turkey and Russia remain close allies. Russia has also cooperated with the YPG, providing air support for anti-Islamic State operations.

The Kurdish outlet Bas News implies Russia is working as an intermediary in the YPG-Turkish dispute, demanding the YPG leave Afrin despite the fact that it falls within the limits of Rojava. “Moscow has given PYD [the political party of the YPG] two options, either to accept the handover of the Afrin region to the Syrian regime, or Turkey will invade it,” Bas News quotes a Kurdish politician in Russia as saying.

The outlet added that the unnamed politician “said that Russia has told PYD that it will protect them from the Turkish invasion on multiple conditions, including obstructing Iranian and US intervention in Afrin, and handing over the city to the Syrian regime.”

Turkey hosted Russian Defense Minister Sergei Shoigu in Ankara this weekend, though the Turkish government did not reveal the specifics of the meeting. Many outlets speculated that the Syrian Civil War was at the top of their list of discussion topics, given Russia’s cooperation with Turkey’s most reviled foes in the country and a recent visit by American envoy to the anti-ISIS coalition Brett McGurk to Raqqa province and Ankara. U.S. officials referred to McGurk’s presence in Syria as part of a “routine” visit. His presence in Ankara, however, followed official calls from Turkey for his firing, claiming he was too supportive of the YPG.