WASHINGTON — Ukraine’s announcement on Thursday of an investigation into alleged surveillance of the U.S. ambassador has come as welcome relief to many in America’s diplomatic corps, but the State Department’s silence on the matter has alarmed a number of current and former diplomats. They are demanding action from the Trump administration.

The department has declined repeated requests to offer any public defense of its envoy, the former U.S. Ambassador to Ukraine Marie Yovanovitch, two days after allegations arose that allies of President Donald Trump had her under illegal surveillance while she was stationed in Kyiv.

The allegations, if true, are central to the impeachment inquiry into Trump, who faces a charge that he abused his presidential power by pressuring Ukraine to investigate Democratic rival Joe Biden, using military aid as leverage. Trump says the inquiry is a “hoax.”

At the time, Trump’s allies were trying to have Yovanovitch, who was seen as a roadblock, removed from her post. She was recalled in May ahead of the end of her tour.

House Democrats this week published text messages to and from Lev Parnas, an associate of Rudy Giuliani, the president’s personal lawyer, suggesting Yovanovitch was under surveillance.

Secretary of State Mike Pompeo, who was traveling in California when the Parnas texts were released, has for months resisted calls for specific statements backing Yovanovitch. One of Pompeo’s top aides, veteran career diplomat Michael McKinley, resigned in October after unsuccessfully pressing for an expression of support for Yovanovitch.

U.S. diplomats have reacted to the new revelations with dismay and to the lack of public support for their colleague with disappointment.

Although those officials all spoke on condition of anonymity, former diplomats and senior lawmakers were outspoken in their criticism, especially after Ukraine announced it would open its own probe into what happened.