Russian Foreign Minister Sergei Lavrov has told U.S. Secretary of State John Kerry that more U.S. military support for forces opposed to Syrian President Bashar al-Assad could stoke violence in the Middle East, the ministry said in a statement on Friday. A U.S. official said on Thursday that President Barack Obama had authorized sending U.S. weapons to Syrian rebels for the first time, after the White House said it had proof the Syrian government had used chemical weapons against opposition forces. During a telephone conversation, Lavrov "stressed that such a step (military support) risks escalating (violence) in the region, while accusations against Damascus of the use of chemical weapons by the United States are not backed up by verified facts," the statement said. This as the White House said it would be dramatically more difficult and costly to set up a no-fly zone over Syria than it was in Libya, stressing that the United States does not have a national interest in pursuing that option. "We feel like the best course of action is to try to strengthen a moderate opposition," Deputy National Security Advisor Ben Rhodes told a news briefing. He said the United States did not want to send U.S. troops - or "boots on the ground" - to Syria and said enforcing a no-fly zone over the country could require intense, open-ended U.S. military engagement. In this regard, President Barack Obama will discuss how to help Syria find ways to end its civil war during the G8 conference in Northern Ireland next week, the White House said.