Yesterday, most of the 400,000 Pentagon staff were ordered to return to work by Defence Secretary Chuck Hagel invoking the Pay Our Military Act.
Congress must act by October 17 in order to avoid a debt default by the US government. The government will run out of cash on that day for the first time in US history unless its debt ceiling is raised.
Republicans who control the House of Representatives have refused to approve the budget, saying they would only do so if the healthcare programme was delayed or stripped of funding.
Obama and the Democrats have refused, noting the law was passed in 2010, subsequently approved by the Supreme Court, and was a central issue in the 2012 election which Obama won.
