WASHINGTON (Reuters) - Republican and Democratic negotiators in the U.S. Congress are closing in on a deal for a stopgap spending bill to fund the government through March 14, averting a partial shutdown that would otherwise begin Saturday, a source familiar with the talks said on Monday.

The measure would likely keep the roughly $6.2 trillion federal budget running at its current level, funding everything from the military to air traffic controllers to federal securities regulators at their current level.

The stopgap measure is needed because Congress failed to pass its one-dozen annual appropriations bills in time for the current fiscal year, which began on Oct. 1. The government's "mandatory" programs, which include Social Security and Medicare retirement and healthcare benefits and represent about two-thirds of the budget, renew automatically.