Turkey was informed of Mr. Trump’s decision, according to an American official who spoke on the condition of anonymity because he was discussing private diplomatic communications.
The Turkish government had no immediate reaction, with officials declining to comment before Mr. Erdogan’s office did.
But the move was likely to anger Mr. Erdogan, who has been pressing the United States to lessen its support for the Syrian Kurds, which the United States considers its most reliable partner against the Islamic State but which Turkey views as terrorists allied with a Kurdish insurgency in Turkey.
While the Kurds are combat-hardened fighters, American officials have said that they will need antitank missiles, heavy machine guns, mortars and armored vehicles to take on Islamic State fighters in Raqqa, who are well equipped and have fortified their positions.
It is not clear how the Trump administration plans to avoid a backlash from Turkey. But American military officials have previously suggested that the United States might provide the Kurdish fighters with just enough weapons to take Raqqa while restricting the supply of arms and ammunition they would receive after that operation ends.
“We would be transparent with them,” Gen. Joseph L. Votel, the commander of the United States Central Command, said of the Turks during a visit to Syria in February. “We could meter things like ammunition.”
Dana W. White, the chief Pentagon spokeswoman, announced the decision in a statement on Tuesday.
“We are keenly aware of the security concerns of our coalition partner Turkey,” Ms. White said. “We want to reassure the people and government of Turkey that the U.S. is committed to preventing additional security risks and protecting our NATO ally.”
In some ways, the announcement formalizes what had already been in practice. The United States has long been working with the Y.P.G., or People’s Protection Units, under the umbrella of the Syrian Democratic Forces. The Pentagon has always emphasized that the S.D.F. includes Arab fighters — and has played down the notion that the United States is working directly with the Y.P.G. — but in reality, the Y.P.G. is believed by both its supporters and its critics to be the dominant force in the S.D.F., providing the most experienced and cohesive fighters.
Still, it is a big step for the American military to be more open and formal about supporting a group with ties to the P.K.K., the militant group driving Turkey’s Kurdish insurgency.
Notably, the Pentagon statement did not list the Y.P.G. by name.
Syria analysts, as well as current and former senior American officials, said Mr. Trump’s decision was not surprising given the military’s insistence on arming the Kurds for the impending battle for Raqqa, but they warned it could damage broader relations with Turkey.
“This decision was probably necessary if the coalition to defeat the Islamic State was to take Raqqa without huge numbers of U.S. troops being directly involved,” said Andrew Exum, a former Army Ranger and top Pentagon official for Middle East policy. “But this decision — to arm a group closely associated with a foreign terrorist organization, and one that has waged a decades-long insurgency against the Turkish state — will likely reverberate through U.S. relations with Turkey for decades to come.”
