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On Wednesday evening, Ukrainian MP Oleksiy Honcharenko said the Ukrainian army had established control over the Sudzha gas hub - a major gas facility involved in the transit of natural gas from Russia to the EU via Ukraine, which has continued despite the war. It is the only point of entry for Russian gas into the EU.
Although this has not been verified by the BBC, Mr Honcharenko’s comment was the first confirmation of an incursion into Russian territory by a Ukrainian official. Kyiv had previously not commented on reports of a cross-border attack.
Thats most likely the main reason, but they don’t get many opportunities like this to slip some people in. It seems like it’d be a waste to not leave behind a team or two to go deeper and gain intel/sabotage things.