Russia’s foreign ministry said on Wednesday the United States had questions to answer over its role in explosions on the undersea Nord Stream gas pipelines last year.

Commenting on a report published earlier on Wednesday that said the United States was involved in the explosions, Russia’s Foreign Ministry spokeswoman Maria Zakharova called on the White House to comment on the “facts” that had been presented.

Reuters was unable to verify the report, published by U.S. investigative journalist Seymour Hersh on his blog, alleging U.S. involvement in the explosions.