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 weapons.