Turkish President Recep Tayyip Erdogan delivers a speech during the Istanbul Youth Festival on May 4. (Ozan Kose/AFP/Getty Images)

Turkey has threatened to step up military action against Syrian Kurdish fighters allied with the United States in response to the Trump administration’s decision to directly arm the Kurds for an assault on the Syrian city of Raqqa, Turkish officials said. 

 The warning was delivered to senior U.S. national security officials in closed-door meetings this week after the Trump administration expressed its intent to arm the Kurds following months of deliberations, the Turkish officials said. 

 “Turkey’s message to the Trump administration was that Turkey reserves the right to take military action,” said a senior Turkish official who spoke on the condition of anonymity because of the sensitivity of the matter.

 Turkey has already conducted limited strikes against the U.S.-backed Kurdish fighters in northern Syria in recent weeks, but it could increase the tempo of those strikes, Turkish officials said. American officials have complained bitterly to Turkey, a NATO ally, about the airstrikes, which have targeted the principal U.S. partner in Syria in the fight against the Islamic State. 

 Any further military action could also potentially complicate the offensive on Raqqa, the Islamic State’s symbolic capital and its last major stronghold after the Iraqi city of Mosul, which is besieged by U.S.-backed Iraqi forces. U.S. officials are concerned that Turkey could send forces into northern Syria and draw the Kurdish fighters away from the Raqqa battle. 

Members of the Syrian Democratic Forces (SDF) walk past collapsed buildings in the northern Syrian town of Manbij on Aug. 7, 2016, as they comb the city a day after they retook it from the Islamic State. (Delil Souleiman/AFP/Getty Images)

Turkish officials reacted with public anger to the U.S. move to arm the Kurds, a decision that was announced Tuesday, a week ahead of a state visit to Washington by Turkish President Recep Tayyip Erdogan.  

Turkey views the U.S.-backed Syrian Kurdish People’s Protection Units, or YPG, as an existential threat. The YPG, which dominates a U.S.-supported force known as the Syrian Democratic Forces (SDF), is affiliated with the Kurdistan Workers’ Party. That organization, known as the PKK, has fought a decades-long insurgency against the Turkish state and is classified as a terrorist group by Turkey and the United States. 

 Erdogan’s government, which had an increasingly testy relationship with Washington toward the end of President Barack Obama’s term, has repeatedly expressed hopes of warmer ties with the Trump administration. In a news conference Wednesday, Erdogan said that Turkey’s “patience has ended” and that he hoped the United States would reverse its decision. 

 “We want to know that our allies will stand not with a terrorist organization but with us,” he said.

 Speaking to reporters in Vilnius, Lithuania, Defense Secretary Jim Mattis said that the United States had “open discussions” with Turkey and that he was confident Ankara’s concerns could be addressed. “It is not always tidy, but we work out the issues,” he said.

In reference to the PKK, Mattis said Turkey was “the only NATO ally” confronting an insurgency on its own ground, and he vowed that the United States would work closely with Turkey to defend its southern border. “It’s Europe’s southern border,” he said, “and we’ll stay closely connected.”

 But from Turkey’s perspective, the U.S. decision amounted to a “very serious crisis” in the relationship, said Ufuk Ulutas, the foreign policy director at the Foundation for Political, Economic and Social Research, a government-friendly think tank in Ankara. 

 The Trump administration has left Ankara with few options other than intensified military action against a Kurdish force that Turkey considers a “direct national security threat,” he said. 

 “Is it going to affect the Raqqa operation? Probably yes,” Ulutas said. “But the problem is, the U.S. is offering nothing — no way to appease Turkey’s security concerns.”

 The Raqqa operation has created a quandary for U.S. military officials, who see the YPG as the group most capable of mounting an assault on the city. American officials have been dismissive of repeated assertions by the Turkish government that it could muster an alternative, and equally effective, military force. 

On Wednesday, the SDF announced that it has seized control of Syria’s largest dam from the Islamic State, as well as the town of Tabqa, the last major obstacle on the road to Raqqa.

 U.S. officials have also tried to assure Turkey that the Kurdish fighters will not play a role in stabilizing the city after the offensive, instead leaving that task to local Arabs. 

A Defense Department official who briefed reporters Wednesday at the Pentagon said that any materiel given to the Syrian Kurds would be carefully monitored to “make sure that it’s being used for exactly the purpose that we intend.” Col. John Dorrian, a spokesman for the coalition command in Baghdad, said the weaponry would include heavy machine guns and unspecified capability to stop vehicle bombs that the Islamic State has used against Iraqi troops fighting to retake the city of Mosul. 

 But verbal assurances were unlikely to persuade the Turkish government, which views the provision of weapons not just as a security threat but also as conferring international legitimacy on a terrorist group, Ulutas said.

 “There will be a lot of complications in bilateral ties,” with implications for security cooperation and regional policies between Turkey and the United States, he said, adding, “Both countries will be in crisis-management mode.”

Entous reported from Washington. Karen DeYoung in Washington and Zakaria Zakaria in Istanbul contributed to this report.