President Barack Obama's fiscal 2016 budget would impose a one-time 14 percent tax on some $2 trillion of untaxed foreign earnings accumulated by U.S. companies abroad and use that to fund infrastructure projects, a White House official said.

The money also would be used to fill a projected shortfall in the Highway Trust Fund.

"This transition tax would mean that companies have to pay U.S. tax right now on the $2 trillion they already have overseas, rather than being able to delay paying any U.S. tax indefinitely," the official said.

"Unlike a voluntary repatriation holiday, which the president opposes and which would lose revenue, the president's proposed transition tax is a one-time, mandatory tax on previously untaxed foreign earnings, regardless of whether the earnings are repatriated."

In the future, the budget proposes that U.S. companies pay a 19 percent tax on all of their foreign earnings as they are earned, while a tax credit would be issued for foreign taxes paid, the official said.