Tensions between Turkey and its historic regional frenemy Iran are rising over Azerbaijan and the arrests of several Turkish citizens accused of links to an Iranian drug baron and the kidnapping of an Iranian dissident in Istanbul.

Sparks began to fly when Turkish President Recep Tayyip Erdogan recited a poem composed by Azeri poet Bahtiyar Vahabzade during an official visit to Baku on Dec. 10 to celebrate Azerbaijan’s victory over Armenia in Nagorno-Karabakh.

A verse that alluded to Iran’s large ethnic Azeri minority was the trigger. “They separated the Aras River and filled it with sand. I will not be separated from you. They have separated us by force,” Erdogan, intoned. The river that rises in Turkey serves as a natural boundary between Azerbaijan and Iran’s Azerbaijan region. The area is home to Turkic-speaking Azeris, who constitute the Islamic Republic’s largest non-Persian minority and are seen as potential separatists prone to exploitation by external enemies.

A day after Erdogan’s recital, Iran summoned Turkey’s Ambassador to Tehran Derya Ors to convey its displeasure. His words were “unacceptable and meddlesome,” the Foreign Ministry said, demanding an explanation. Iran’s Foreign Minister Mohammad Javad Zarif went further, asserting via Twitter that the poem “refers to the forcible separation of areas north [of the] Aras from [the] Iranian motherland. Didn’t he realize he was undermining the sovereignty of the Republic of Azerbaijan?"