WASHINGTON - The US State Department on Tuesday said it had no reason to believe Syrian government charges that rebels had used chemical weapons in the country's civil war, but said it was still studying allegations from the opposition that Syrian armed forces had used the weapons.Department spokeswoman Victoria Nuland called the charges an effort by Syrian President Bashar Assad's government to discredit its opponents.Nuland, portraying the Assad government as increasingly beleaguered, said Washington is "quite concerned" that it would resort to non-conventional weaponsMeanwhile, the White House on Tuesday said it was looking carefully at allegations that chemical weapons were used in Syria, but said it had no evidence to substantiate charges that the opposition had used such weapons."We are looking carefully at the information as it comes in," White House spokesman Jay Carney told reporters. "This is an issue that has been made very clear by the president to be of great concern to us."Meanwhile, Britain's UN envoy said on Tuesday that reports of a chemical weapon attack in Syria had not yet been "fully verified" as the Russian Foreign Ministry accused Syrian rebels of using chemical weapons near the northern city of Aleppo.Syria's government and rebels accused each other of launching the deadly chemical attack on Tuesday. If confirmed it would be the first use of such weapons in the two-year-old conflict."We have seen those reports, they haven't yet been fully verified," Britain's UN Ambassador Mark Lyall Grant told reporters on his way into a UN Security Council meeting on Afghanistan."But clearly if chemical weapons were used then that would be abhorrent and it would require a serious response from the international community," he said.The Security Council has been deadlocked on Syria since 2011. Russia and China have refused to consider sanctions on President Bashar Assad's government, and have vetoed three resolutions condemning Assad's crackdown on opposition groups.The conflict began as peaceful protests that turned violent when Assad tried to crush the revolt. The United Nations says more than 70,000 people have been killed and more than 1 million people have fled the violence.