(Reuters) - The Washington Monument will remain closed to visitors until technicians can overhaul its elevator system after a series of breakdowns at one of the U.S. capital's most popular tourist attractions, the National Park Service said on Monday.

The 555-foot (169-meter) obelisk, erected on the District of Columbia's National Mall in honor of President George Washington in the 19th century, had been scheduled to reopen in mid-September after its most recent closure began on Aug. 17.

"Despite the continuing work on the Washington Monument elevator, we have not been able to determine the causes of the ongoing reliability issues," the National Park Service, which oversees the monument, said in a statement.

"As a result, we have made the difficult decision not to reopen the Washington Monument until we can modernize the elevator control system."

The agency said it expected to announce the duration of the closure in the next few weeks.

Electrical short-circuiting, mechanical and computer problems have forced the closure of the world's tallest stone structure about two-dozen times since it reopened in May 2014 following repairs for earthquake-related damage sustained in 2012.

The current closure was caused by a cable breaking loose, Park Service spokesman Mike Litterst said.

The monument, which is also the world's tallest obelisk, was closed in July to replace a pulley bearing, and shut down in May because of a faulty computer chip in a control panel, Litterst said.

In an incident in August, tourists in the monument's elevator were forced to exit using stairs because the elevator doors failed to open when electrical issues tripped circuit breakers.

Work will continue on the monument, which attracts about 600,000 visitors a year, for an unknown period of time, the Park Service said.


By Laila Kearney