WASHINGTON — In a contentious hearing before Congress, a senior official from the Department of Veterans Affairs’ watchdog agency acknowledged for the first time on Wednesday that delays in care had contributed to the deaths of patients at the department’s medical center in Phoenix.

The disclosure by an official from the department’s inspector general’s office, coming after more than two hours of tough, sometimes confrontational exchanges with members of the House Veterans Affairs Committee, was a significant development in what has become a heated dispute over the quality of care at the Phoenix hospital, where revelations of secret waiting lists and other schemes to disguise long delays in care turned into a national scandal.

Republicans characterized the acknowledgment as an about-face, and expressed frustration and some anger that a report on the Phoenix hospital issued by the inspector general last month contained language widely viewed as playing down concerns about a link between the medical-care delays and veterans’ deaths.

As the waiting-list scandal began to break in Phoenix this spring — and soon became a national controversy that led to the ouster of the department’s secretary, Eric Shinseki, and the suspension of the hospital’s director — claims were made by whistle-blowers and on Capitol Hill that the deaths of as many as 40 veterans could be attributed to delays in care.