President Barack Obama has accused his critics of extorting him over raising the US debt ceiling, as a Republican budget gamble brought the prospect of a government shutdown ever closer. Obama and his Republican foes in Congress are locked into entrenched positions over lifting the $US16.7 trillion ($A17.9 trillion) borrowing cap and a government operating budget. If there is no compromise - an increasingly likely prospect - government operations will grind to a halt on October 1 and the US could default on its debts by the middle of next month, a scenario which could have alarming domestic and international...