UNITED NATIONS (AP) — Arming Syrians fighting on either side of the country's bloody conflict could lead to more war crimes, the head of the U.N. commission investigating human rights abuses in Syria warned Friday.

Paulo Pinheiro stopped short of directly criticizing the United States or other nations arming Syrians, saying the commission does not comment on decisions by governments. But his remarks came about a week after President Barack Obama authorized sending weapons to rebels for the first time.

It marked a major policy shift for the Obama administration, and came after the White House disclosed that the U.S. had conclusive evidence that Bashar Assad's regime had used chemical weapons against the opposition trying to overthrow him.

"States who provide arms have a responsibility in terms of the eventual use of these arms to commit gross human rights violations, war crimes or crimes against humanity," Pinheiro said. "We are very much worried that more arms will signify an increased presence of violations and those crimes."

The U.N. Commission of Inquiry earlier this month in a report to the U.N. Human Rights Council in Geneva accused both sides of committing war crimes in Syria. About 93,000 people have been killed in Syria since the first protests began in March 2011.

"Crimes that shock the conscience have become reality ...We emphasize that there is a devastating human cost to the availability of weapons and that there is also a political cost," Pinheiro said. "Weapons fuel the parties' illusion that they can win this war, pulling them farther into battle and away from the negotiating table."

He reiterated his support for a Syria peace conference called for by Russia and the United State that would bring together rebels of the Free Syrian Army and forces of President Bashar Assad. No date for those talks has been set, but leaders at the Group of Eight who met this week in Northern Ireland all agreed talks should start soon.

In its report, the commission also found there were "reasonable grounds" to believe that limited quantities of toxic chemicals had been used as weapons in at least four attacks in Syria's civil war, but that more evidence was needed to determine the precise chemicals agents used, or who used them.

The commission said conclusive findings could only be reached by getting testing samples directly from victims of the attack, and U.N. chemical weapons experts still have not been allowed to enter Syria. Pinheiro, who briefed the U.N. Security Council on Friday, reiterated that U.N. investigators still did not have enough evidence to show the precise chemical agents being used in Syria, and who was using them.

In Syria, meanwhile, rebels said they had received shipments of new weapons from Arab nations and other countries, though none yet from the U.S.