By Phil Stewart and David Dolan
BRUSSELS/ISTANBUL (Reuters) - The U.S. defense secretary said on Wednesday Syria appeared to have heeded a warning from Washington not to carry out a chemical weapons attack.
But Russia, the Syrian government's main backer, said the U.S. assertions that President Bashar al-Assad's forces may have been planning a chemical attack complicated peace talks on ending Syria's six-year-old civil war.
The White House said on Monday it appeared the Syrian military was preparing to conduct a chemical weapons attack and Assad and his forces would "pay a heavy price" if it did so.
U.S. officials later said the warning was based on intelligence that indicated preparations for such a strike were under way at a Syrian airfield.
"It appears that they took the warning seriously," U.S. Defense Secretary Jim Mattis said on Wednesday.
"They didn't do it," he told reporters flying with him to Brussels for a meeting of NATO defense ministers.
He offered no evidence other than the fact that an attack had not taken place.
Asked whether he believed Assad's forces had called off any such strike completely, Mattis said: "I think you better ask Assad about that."
Washington accused Syrian forces of using the airfield, the Shayrat Airbase, for a chemical weapons attack in April. Syria denies this.
However, Mattis said Syria's chemical weapons threat was larger than any single location.
"I think that Assad's chemical program goes far beyond one airfield," he said.
U.S. and allied intelligence officers had for some time identified several sites where they suspected Assad's government may have been hiding newly made chemical weapons from inspectors, a U.S. official familiar with the intelligence said.
The United States launched a cruise missile strike on Shayrat Airbase in April following the deaths of 87 people in what Washington said was a poison gas attack in rebel-held territory.
The Syrian military and Foreign Ministry did not comment on the White House warning although state-run al-Ikhbariya television station said the allegations were fabricated.
Russia denounced the warning and dismissed White House assertions that a strike was being prepared as "unacceptable".
Russian Deputy Foreign Minister Gennady Gatilov on Wednesday told the United States not to take unilateral actions in Syria.
He said the U.S. assertions complicated peace talks on Syria, according to RIA news agency.
Russian officials have described the war in Syria as the biggest source of tension between Moscow and Washington and the April cruise missile strike ordered by U.S. President Donald Trump raised the risk of confrontation between them.
In Washington, the U.S. ambassador to the United Nations, Nikki Haley, credited Trump with saving Syrian lives.
"Due to the president's actions, we did not see an incident," Haley told U.S. lawmakers. "I would like to think that the president saved many innocent men, women and children."
Although the number of people killed in suspected chemical attacks is a small portion of the total dead in Syria's civil war -- estimated at close to half a million -- footage of victims writhing in agony has caused particular revulsion.
DISPUTE OVER YPG
On the Syrian battlefields, Turkish artillery bombarded and destroyed Kurdish YPG militia targets after the group's fighters opened fire on Turkish-backed forces in northern Syria.
The United States supports the YPG in the fight against Islamic State in Syria, while NATO ally Turkey regards them as terrorists indistinguishable from militants from the outlawed PKK (Kurdistan Workers Party), which is carrying out an insurgency in southeast Turkey.
The Turkish army said YPG machinegun fire on Tuesday evening targeted Turkish-backed Free Syrian Army rebels south of the town of Azaz. Artillery struck back in retaliation, a Turkish military statement said.
The boom of artillery fire could be heard overnight from the Turkish border town of Kilis, broadcaster Haberturk said.
Ankara was angered by a U.S. decision in June to arm the YPG in the battle for Islamic State's stronghold of Raqqa.
Secretary Mattis on Tuesday left open the possibility of longer-term assistance to the YPG, saying the United States may need to supply them weapons and equipment even after the capture of Raqqa.
Turkey last year sent troops into Syria to support Free Syrian Army rebels fighting both Islamic State and Kurdish forces who control a large part of Syria's northern border region. President Tayyip Erdogan has said Turkey would not flinch from taking tougher action against the YPG in Syria if Turkey believed it needed to.
In Geneva, the United Nations human rights chief said at least 173 civilians have been killed in air and ground operations against Islamic State in Raqqa this month.
Zeid Ra'ad al-Hussein urged all sides involved to take all possible precautions to spare the lives of 100,000 civilians still trapped.
"Civilians must not be sacrificed for the sake of rapid military victories," Zeid said in a statement.
(For a graphic on map of Syria showing areas of control, click http://tmsnrt.rs/2stEWIz)
(Additional reporting by Orhan Coskun, Tulay Karadeniz and Omer Berberoglu, Editing by Angus MacSwan)