MOSCOW — Russia will deliver its S-400 air-defense missile system to NATO member Turkey in the coming days, a Kremlin spokesman said Friday, in a deal likely to trigger U.S. sanctions and test the bonds of the Western military alliance.

But the scope of the possible response from Washington remains clouded by apparent conflicting messages.

President Donald Trump has publicly shown sympathy for Turkish President Recep Tayyip Erdogan’s position on the Russian missile purchase. Secretary of State Mike Pompeo, however, has warned of tough measures that could include canceling the sales of F-35 fighter jets to Turkey.

For the wider NATO alliance, the Turkish deal strikes at the heart of military coordination. NATO has expressed worry that the S-400 is incompatible with its possession of the U.S.-made F-35s, and would give Russia access to secrets of its stealth technology.

For more than a year, the United States has urged Erdogan not to procure the sophisticated Russian air-defense system — a move that would bring mandatory U.S. sanctions against Turkey under a 2017 law on cooperation with “adversaries.”

The U.S. measures, if carried out, would cause an extraordinary breach in U.S.-Turkey relations and almost certainly complicate ongoing negotiations between the two countries over other issues, including military strategy in Syria.

But while U.S. officials have portrayed the sanctions against Turkey as a matter of certainty, Trump refrained from taking a hard line last month at the Group of 20 summit in Japan.

Trump, speaking to reporters at the summit, said Erdogan had first sought to buy Patriot missiles but had been “treated very unfairly” by the Obama administration, without giving specifics.

But the United States had attempted to strike a deal with Ankara over the Patriot systems. Erdogan insisted that any deal include sharing technology so that Turkey can develop and build its own missiles. The Obama administration declined the offer.

At the G-20 summit, Trump did not directly answer questions about whether the United States would impose sanctions on Turkey.

Erdogan, however, said after his talks with Trump that Turkey would be spared the U.S. sanctions.

Erdogan has portrayed the purchase as Turkey’s sovereign right and said that the United States failed to offer a comparable deal for the Patriot missile system.

Asked by a reporter during a telephone briefing if Russia would deliver the system on Sunday, Kremlin spokesman Dmitry Peskov said, “I can confirm on behalf of the Kremlin that the S-400 deal is going according to plan.”