BEIRUT (AP) ― Turkey and Syria traded artillery fire for the fourth day in a row Saturday as rebels clashed with President Bashar Assad’s forces near the border, heightening fears that the crisis could erupt into a regional conflict.

Also Saturday, Syrian Defense Minister Gen. Fahd Jassem al-Freij vowed to crush the rebellion and bring the violence that has engulfed the country to an end.

The diplomatic crisis began on Wednesday, when a Syrian shell killed five civilians in a Turkish border town and triggered unprecedented artillery strikes by Turkey. Ankara has deployed more troops to its southern border with Syria, and has responded to each shell that has struck Turkish soil with its own artillery barrage.

On Friday, Turkish Prime Minister Recep Tayyip Erdogan cautioned Damascus not to test Turkey’s “limits and determination” and said Ankara was not bluffing in saying it won’t tolerate such acts.

Saturday’s cross-border exchange began when two mortar shells fired from Syria landed in rural areas near the Turkish village of Guvecci, prompting Turkish return fire, Turkey’s media reported.

Later Saturday, a third shell hit near another village in Turkey’s Hatay province and Turkish troops fired back, the office of the provincial governor said.

No casualties were reported.

The first exchange happened shortly after intense fighting broke out across the border in Syria’s Idlib province between Syrian rebels and regime forces, the private Dogan news agency reported.

A Turkish army unit based near Guvecci fired four 81 mm mortar shells in the first instance and two shells in the second, it said. No casualties were reported.

The Hatay governor’s office indicated that the Syrian mortar had landed in Turkey accidentally, saying it was believed “to be have been fired by the forces of the Syrian Arab Republic at Syrian rebel groups on the Syrian side of the border.”

The Britain-based Syrian Observatory for Human Rights said rebels had attacked army positions in the Syrian villages of Khirbet al-Jouz and Darkoush about 16 kilometers from Guvecci. Observatory director Rami Abdul-Rahman said both sides were exchanging mortar fire.