This photo released by the press office of the Kurdish militia, People's Protection Units or YPG, shows protesters waving giant flags of the YPG and other parties and militias, during a demonstration against Turkish threats, in Afrin, Aleppo province, north Syria, Thursday, Jan 18, 2018. Deputy Syrian Foreign Minister Faysal Mekdad said the country's air force will shoot down any Turkish fighter jets that attack Syria. The warning comes as tensions are skyrocketing over apparent Turkish preparations to attack a Kurdish enclave in the north of the country. (YPG Press Office via AP)
This photo released by the press office of the Kurdish militia, People's Protection Units or YPG, shows protesters waving giant flags of the YPG and other parties and militias, during a demonstration against Turkish threats, in Afrin, Aleppo province, north Syria, Thursday, Jan 18, 2018. Deputy Syrian Foreign Minister Faysal Mekdad said the country's air force will shoot down any Turkish fighter jets that attack Syria. The warning comes as tensions are skyrocketing over apparent Turkish preparations to attack a Kurdish enclave in the north of the country. (YPG Press Office via AP) Credit: Uncredited

Syria said on Thursday its air defense would shoot down any Turkish jets that carry out attacks within Syria, a stark warning as tensions soar over apparent preparations by the Turkish military to invade a northern Syrian Kurdish enclave.

From Damascus, Deputy Foreign Minister Faysal Mekdad said a military incursion into Syria’s Kurdish-controlled enclave of Afrin would be “no picnic” for Turkey.

Any such operation would be considered an “aggressive act,” Mekdad said.

The escalation comes after the United States disclosed plans to form a 30,000-strong border force in Syria led by the Kurdish-dominated Syrian Democratic Forces in the wake of the victory over the Islamic State group. Turkey reacted angrily to the announcement.

Turkey regards the Syrian Kurdish militia that controls Afrin and other areas along its frontier as an extension of the Kurdish insurgency within its borders and wants to prevent the establishment of a Kurdish corridor along the frontier.

The U.S. has developed close ties with the Kurdish militia over the shared fight against ISIS and keeps bases in northeast Syria with the Kurds. But it is not believed to maintain any American forces in Afrin, an enclave that is not linked to the bulk of Kurdish-run territory in Syria.

Earlier on Thursday, Turkish Prime Minister Binali Yildirim called on the U.S. to abandon ideas of a Syrian Kurdish border force, saying that Washington has been inconsistent in its statements about its plans.

Turkish President Recep Tayyib Erdogan had called the U.S.-proposed force an “army of terror,” and vowed to crush it. He said Turkey would launch a military offensive against the enclave of Afrin and other Syrian Kurdish militia-controlled territories, and was massing troops and tanks on its border.

In Afrin, residents took to the streets to protest Turkey’s threats, according to photos published by the Kurdish news agency Hawar. They waved flags of the Syrian Kurdish militia, the YPG, and also banners of Abdullah Ocalan, the imprisoned leader of the outlawed Kurdistan Workers’ Party, or PKK, which Turkey considers a terrorist organization.

Mekdad, the Syrian deputy foreign minister, told reporters in Damascus that Syria’s air force has been restored to its former might after seven years of civil war and was “ready to destroy Turkish air targets in the skies of the Syrian Arab Republic.”

Syria’s air force suffered a number of plane and helicopter losses in the government’s war on opposition areas inside the country and the Islamic State group. It has been unable to stop Israeli airstrikes against suspected Hezbollah targets within Syria.