ISTANBUL — 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 US national security officials in closed-door meetings this week, after the Trump administration expressed its intent to arm the Kurds after months of deliberations, the Turkish officials said.

‘‘Turkey’s message to the Trump administration was that Turkey reserves the right to take military action,’’ a senior Turkish official said.

Turkey has already conducted limited strikes against the US-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 US 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 US-backed Iraqi forces. US officials are concerned that Turkey could send forces into northern Syria and draw the Kurdish fighters away from the Raqqa battle.

Turkish officials reacted with public anger to the US 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 US-backed Syrian Kurdish People’s Protection Units, or YPG, as an existential threat. The YPG, which dominates a US-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 both Turkey and the United States.

Erdogan’s government, which had an increasingly testy relationship with Washington toward the end of President Obama’s term, has repeatedly expressed hopes of warming ties with the Trump administration. In a news conference Wednesday, Erdogan said 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 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.

But 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,’’ said Ufuk Ulutas, the foreign policy director at the Foundation for Political, Economic and Social Research, a government-friendly think tank in Ankara.

‘‘Is it going to affect the Raqqa operation? Probably yes. But the problem is, the US is offering nothing — no way to appease Turkey’s security concerns,’’ he said.