June 1 remains a "difficult deadline" to raise the U.S. debt ceiling, and unless the standoff with Republicans in Congress is resolved, the government won't be able to pay its bills halfway through next year.

This was said on Sunday by the US Treasury Secretary, Janet Yellen. "I think it's a tough deadline," Yellen told NBC's Meet the Press. The government could run out of money and default as early as June 1 if Congress, where Republicans control the House of Representatives, does not authorize further lending.