Media reports say a colonel serving in Ukraine's special operations forces played a central role in the attack on the Nord Stream natural gas pipelines last year.
The bombings of the Nord Stream 1 and 2 pipelines in September last year caused massive gas leaks. Running under the Baltic Sea, the pipelines link Russia and Germany.
The Washington Post reported on Saturday that the colonel was the "coordinator" of the covert operation. It cited officials in Ukraine and Europe, as well as others familiar with the details.
The report said he managed logistics and support for a six-person team that placed explosive charges on the pipelines.
It also said he took orders from more senior military officials, who ultimately reported to Ukraine's Commander-in-Chief, General Valerii Zaluzhnyi. The report said Ukrainian President Volodymyr Zelenskyy may have been deliberately kept uninformed of the operation.
The colonel is currently under detention in an unrelated case. He has reportedly said through an attorney that he did not play any role in the pipeline blasts.
Reuters has quoted a spokesperson for Ukraine's military as saying on Sunday that he had "no information" about the report.
Russian presidential spokesperson Dmitry Peskov said on Monday that "traces of Ukraine in this sabotage" are increasingly appearing in investigations and media reports. Peskov said Russia is watching carefully.