Turkish President Recep Tayyip Erdogan speaks during a news conference to present the outcome of the G20 leaders summit in Hamburg, Germany July 8, 2017. REUTERS/Wolfgang Rattay

ISTANBUL (Reuters) - President Tayyip Erdogan said on Saturday Turkey would not watch passively as weapons are sent to Kurdish fighters on its southern border, saying his country would respond to any threats to national security.

The United States has been arming Kurdish YPG fighters taking part in the battle to recapture the Syrian city of Raqqa from Islamic State, angering its NATO ally Turkey. Ankara views the YPG as an extension of the Kurdish PKK group that has waged a long insurgency in southeast Turkey.

Turkey, Washington and the European Union have all designated the PKK as a terrorist organization.

“We will definitely not remain silent and unresponsive to the support and arming of terror organizations next to our borders and the forming of terror islands in the region,” Hurriyet Daily News website quoted Erdogan as saying.

“We will not hesitate to use our right to self defense against formations threatening the security of our country,” Erdogan told a news conference at the G20 summit in Hamburg, according to the web site.

Erdogan expressed Turkey’s alarm at the U.S. decision to arm the YPG at a White House meeting with U.S. President Donald Trump in May. The two men also met at the G20 summit.

Syrian rebels said on Friday they were preparing to join the Turkish military in a major new offensive against Kurdish forces in northwestern Syria, raising the prospect of yet another front in an increasingly complex conflict.

Turkish officials have not commented on any military preparations in northern Syria. Turkish troops launched an incursion across the border last August in support of Syrian rebel fighters, targeting both Islamic State and the YPG.