Syria has threatened to shoot down Turkish warplanes in its airspace, as Turkey prepares to carry out a possible assault on a Kurdish enclave.
Deputy Foreign Minister Faisal Mekdad said any incursion into Afrin would be considered an act of aggression.
It came as Turkey sought clearance for an air campaign from the Syrian government’s allies, Russia and Iran.
Tanks are already massed on the border near Afrin, which is held by Kurdish fighters Turkey considers terrorists.
Kurdish leaders have promised to defend the enclave and urged the international community to prevent Turkey targeting what they said were its one million residents.
Turkey has for months said it would clear fighters from the Syrian Kurdish People’s Protection Units (YPG) from Afrin. It sees the militia as an extension of the banned Kurdistan Workers’ Party (PKK), which has fought for Kurdish autonomy in Turkey.
The YPG denies any direct links – an assertion backed by a US-led coalition whose air strikes and special forces have helped the militia and allied Arab fighters drive Islamic State militants from tens of thousands of square kilometres of Syria.
Turkey appeared to bring forward plans for assaults on Afrin and Manbij, another Kurdish-controlled area further east, when coalition officials said on Saturday that it was helping the YPG and its allies form a 30,000-strong “border security force”.
US Secretary of State Rex Tillerson later said the officials “misspoke”. He explained that the US was not creating a new force. Rather, it was trying to ensure allied fighters were able to protect their territory from attacks by IS remnants.
But his Turkish counterpart, Mevlut Cavusoglu, said Ankara was not fully satisfied.
“The establishment of a so-called ‘terror army’ would cause irreversible damage in our relations,” he told CNN Turk on Thursday. “It is a very serious situation.”
Mr Tillerson’s comments failed to end the build-up of Turkish tanks and troops in border areas west and north of Afrin. Turkish soldiers have also been shelling the enclave from positions to the south in the rebel-held Syrian province of Idlib.
On Thursday, Turkey’s military and intelligence chiefs flew to Moscow to persuade their Russian counterparts to allow Turkish warplanes to support any offensive.
Despite supporting opposing sides of the conflict in Syria, Turkey and Russia have co-operated on the creation of “de-escalation zones” in Idlib and three other areas.
“We are meeting the Russians and Iran on the use of airspace,” Mr Cavusoglu said. “The Russians should not oppose an Afrin operation.”
But he noted that they needed to co-ordinate over a small number of Russian military observers already on the ground in Afrin “to avoid an accident”.
Mr Cavusoglu accused YPG fighters in the enclave of attacking Turkish territory every day and insisted that it was his country’s “right to self-defence in line with international law to take measures against a terror group”.
But Syria’s deputy foreign minister warned that a Turkish military incursion would be “no picnic” and that the Syrian Air Force was “ready to destroy Turkish air targets in the skies of Syria”.
“We warn the Turkish leaders that if they start fighting in the region of Afrin, it will be seen as an aggression by the Turkish army against the sovereignty of Syria,” Mr Mekdad added.
In Afrin, thousands of residents participated in a protest against the Turkish threats and shelling of the enclave, the Kurdish Anha news agency reported.