WASHINGTON (Reuters) - President Barack Obama raised the stakes in the third straight day of budget talks on Tuesday, warning that senior citizens and veterans may suffer first if the debt ceiling is not raised by August 2.

The comments came as top Republicans toughened their stance in the deficit reduction talks.

Hours before heading to the White House, Senate Republican leader Mitch McConnell said a "real solution" to the U.S. debt problem was unlikely while Obama was in office.

As politicians sparred, U.S. business leaders pressed Obama and congressional leaders to act swiftly to raise the $14.3 trillion U.S. debt ceiling or risk derailing a sputtering economic recovery and endangering the global financial system.

Obama and top lawmakers from both parties began their third day of talks in as many days at the White House at 3:55 p.m. EDT. The two sides are seeking to break a logjam over the role of taxes in a deficit-cutting deal.

Obama said in an interview with U.S. television network CBS before the meeting that checks to recipients of the Social Security retirement program may not go out in early August if he and congressional leaders do not agree on a debt deal.