Defense Secretary Jim Mattis is welcomed by Lt. Gen. Stephen Townsend upon his arrival in Baghdad on Feb. 20. (Thomas Watkins/AFP/Getty Images)

Forces combating the Islamic State in Syria should focus on defeating the militants rather than wasting energy and resources fighting among themselves, the head of the U.S.-led coalition in Iraq and Syria said Wednesday.

Lt. Gen. Stephen Townsend said that Kurdish fighters would participate “in some form or fashion” in the upcoming operation to retake the city of Raqqa, the Islamic State’s de facto capital in Syria. But he insisted that they will largely be “local Kurds” from the Raqqa area who pose no threat to neighboring Turkey.

Turkish President Recep ­Tayyip Erdogan said this week that Turkish forces would participate in the offensive but that any involvement by the Syrian Kurdish People’s Protection Units, or YPG, was unacceptable. Turkey considers the YPG, which has been in the forefront of U.S.-backed ground operations in Syria, to be a terrorist organization in league with its own separatist Kurds and responsible for terrorist attacks inside Turkey.

Townsend said he has “seen absolutely zero evidence” of that. Pentagon officials have been negotiating with Turkey for weeks in an effort to include its forces in the Raqqa offensive while not throwing out existing plans for YPG participation.

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“We encourage all forces to remain focused on the counter-ISIS fight and concentrate their efforts on defeating ISIS and not toward other objectives that may cause the coalition to divert energy and resources away from Raqqa,” Townsend told reporters via a video link from Baghdad. ISIS is an acronym for the Islamic State.

Discussions over how and when to begin the operation have continued as the Pentagon this week delivered its updated plan for that and other counter-Islamic State operations in Syria and Iraq. Townsend said he had provided his own recommendations for the plan but declined to discuss them until the White House has finished its deliberations.

Asked about expectations that the administration will send more troops to Syria and Iraq to supplement the current 500 in the former and 5,000 in the latter, he said, “I don’t foresee us bringing in large numbers of coalition troops, mainly because what we’re doing is, in fact, working.”

But he acknowledged the challenges posed by the Raqqa operation, as well as the larger problems of the crowded and chaotic battlefield in northwestern Syria.

“Just this week . . . last week, we saw Turk and Turk proxy forces converge with Syrian regime and Syrian proxy fighters, ISIS being in the mix there,” Townsend said. “We have YPG, Syrian Democratic Force fighters and Syrian Arab Coalition fighters also bumping right up against each other there. And then here in the last 48 hours, we’ve seen Syrian regime forces advance through ISIS-held villages to essentially rifle-range or hand-grenade range with Syrian Arab Coalition fighters holding the area around Manbij.”

The Syrian Democratic Forces are rebels participating in Turkey’s military advance south from its border with Syria toward the Islamic State-held town of al-Bab. The Syrian Arab Coalition is the joint Kurdish-Arab group moving south toward Raqqa. Both are being assisted by U.S. Special Operations forces and U.S. warplanes.

Meanwhile on Tuesday, Townsend revealed, Russian and Syrian aircraft aiding the Turkish advance bombed “our Syrian Arab Coalition forces” between al-Bab and Manbij, causing some rebel casualties.

He said he believed that the pilots thought they were bombing the Islamic State. U.S. forces, just a few miles away, called in the information and halted the airstrikes, Townsend said. Moscow later denied that Russian or Syrian aircraft had bombed the area.

The United States and Russia established a hotline in 2015 in which a U.S. colonel in Qatar and a Russian colonel in Syria work to “deconflict” operations and prevent aerial collisions. Senior U.S. officials have not cooperated directly with the Russians, in part because of 2014 legislation that forbids engagement because of Russia’s annexation of Ukraine’s Crimean Peninsula.

Senior U.S. military officials in the Middle East have advocated for increased deconfliction talks, citing the proximity in which the militaries are operating in Syria. But the military has been widely opposed to President Trump’s proposed cooperation with Russia in the broader counterterrorism fight.

At a NATO conference last month, Defense Secretary Jim Mattis accused Russia of violating international law with “aggressive” and “destabilizing” actions, including interference in U.S. and other elections, and said the United States is “not in a position right now to collaborate on the military level.”

Russia has been the primary backer of Syrian President Bashar al-Assad in his separate conflict against opposition forces fighting a civil war against him with backing from the United States and European and regional powers.

In a report issued Wednesday in Geneva, a special U.N. investigative commission said that all sides in the civil war committed war crimes during the battle for Aleppo late last year, including the Syrian government’s deliberate bombing of a humanitarian convoy and use of chemical munitions.

Syrian warplanes, it said, “meticulously planned and ruthlessly carried out” the Sept. 19 attack, killing 10 aid workers who were preparing to deliver humanitarian assistance to besieged civilians during a supposed cease-fire.

Both Syria and Russia had denied the attack.