Federal regulators yesterday took over financially ailing United Savings Bank, saying the Vienna-based savings and loan had been weakened by the sharp slowdown in the Washington area's real estate market and by a three-day run on deposits by its customers.

United is the first local savings and loan to fail during the current crisis in the nation's thrift industry and the first to be taken over since a series of state-chartered Maryland S&Ls failed in 1985. Unlike those failures, however, in which some accounts were frozen for years, customers will continue to have access to their deposits at United, and all of its 15 branches in Northern Virginia will be open as usual, federal authorities said yesterday.

United will reopen today as United Federal Savings Bank under the management of the Resolution Trust Corp., the government agency charged with handling the thrift cleanup. RTC officials, seeking to soothe jittery depositors, noted that United's deposits remain federally insured up to the limit of $100,000.

A team of officials from the RTC and Office of Thrift Supervision walked into United's headquarters in Vienna late yesterday afternoon and presented documents to United's senior management notifying them of the takeover. After signing the papers, United President Stanley Burns immediately left the office and headed home, said an RTC official, who described the transition as orderly. Burns was unavailable for comment.

Burns is the only United employee who will be replaced, officials said. The S&L will be overseen by RTC managing agent Ronald Falls until the government can find a buyer or dispose of United's assets.

Although several local banking institutions recently reported steep losses as a result of a decline in the local real-estate market, federal officials say thrifts in the area are relatively healthy compared to the industry as a whole. Previously, regulators have said that one S&L in the District of Columbia, eight in Maryland and eight in Virginia are in danger of failing out of appropriately 200 institutions in the three jurisdictions. The troubled thrifts were not identified and it isn't known whether United was among them.

The government said it stepped in at United because the thrift was operating in "an unsafe and unsound condition" that threatened its assets and deposits and ultimately could have deepened the drain on the federal deposit insurance fund. It attributed United's deteriorating condition to "an excessive concentration of poorly underwritten commercial real-estate loans" and construction and development loans. The RTC also termed the S&L's policies for evaluating and reviewing loans "inadequate."

Under previous management in the mid-1980s, United grew rapidly by attracting large deposits with higher-than-average interest rates. It lent these funds primarily to commercial developers, many of whom have had trouble repaying the loans due to the recent slowdown in the Washington-area real estate market.

The government attempted to stop United's slide in May last year by placing it under a supervisory agreement that restricted how much and to whom it could lend money.

But the thrift's losses continued to mount nonetheless, going from $6.5 million for all of 1989 to $11.5 million during the first six months of this year. United's announcement that it lost $9.8 million during the second quarter and was nearly insolvent touched off a run on deposits that continued into a third day yesterday, prompting the federal action.

RTC spokeswoman Nancy Schertzing said there was no immediate way of determining how much the failure of United would cost taxpayers until the institution is stabilized.

The government said United had $415.6 million in assets and $414.9 million in liabilities as of June 30 and had a capital deficit of $2.97 million.

The biggest immediate losers in United's demise are its shareholders, whose investment is wiped out by the government takeover.