A refusal by U.S. oil companies including Exxon Mobil Corp. (NYSE: XOM) and Chevron Corp. (NYSE: CVX) to disclose their U.S. tax payments is undermining the international effort to fight corruption in natural resources industries worldwide, according to transparency campaign groups.

The chairman of the Extractive Industries Transparency Initiative (EITI), whose participants include 51 governments and most major western companies, said it was “disappointing” that the largest U.S. oil groups “did not provide the leadership expected from them,” in the latest sign that the 15-year-old initiative is under strain.

The Trump administration withdrew the U.S. from the initiative in November, even though it said at the time it continued to “value the EITI as a critical tool to promote transparency, increase competitiveness, and combat corruption around the world.”