WASHINGTON (AP) — President Barack Obama's administration insists that Syrian President Bashar Assad's government carried out a chemical weapons attack last week against his own people, the latest turn in an ongoing conflict in the country. Assad's government denies the allegations and insists the rebels are to blame for any chemical attack, and Russia is among the countries lining up behind him. There are fears that Assad's stockpile of chemical weapons, believed to be one of the world's largest, could fall into the hands of Islamic extremist groups. [...] in June, the White House said it had conclusive evidence that Assad used chemical weapons against rebel fighters, and Obama decided to respond by authorizing the arming of Syria's rebels. The civil war is a complicated and brutal one with heavy civilian casualties on both sides even before last week's alleged chemical attack. By mid-2011, a loose coalition of rebels and anti-government tribal groups formed the Free Syrian Army whose goal was to topple the Assad regime. In some towns and cities, activists accused regime forces of execution-style killings and a scorched earth campaign of burning homes. Obama called the alleged chemical attacks "a challenge to the world" and says he has an "obligation" to act and is considering a "limited, narrow act" to send a strong message to Syria and others who might consider using chemical weapons. Even before the alleged chemical attacks, the Assad regime was hit with an increasing number of sanctions from European countries and the U.S.