ZUHRES, Ukraine — Just hours after the Malaysian government reached an agreement with Ukrainian separatists on Sunday over access to the crash site of a Malaysian airliner shot down in rebel territory, the Ukrainian military launched an operation to recapture the debris fields, again stalling international efforts to secure the site.

The heavy fighting threatened to torpedo hopes of a breakthrough and cause yet more delays in collecting evidence and retrieving the remaining bodies from the crash. Ukrainian security officials said they needed control over the site to prevent the pro-Russia separatists from destroying clues to the downing of Malaysia Airlines Flight 17.

By Sunday evening, the Ukrainian advance had blocked a road leading from the provincial capital, Donetsk, to the airplane debris northeast of Shakhtyorsk, but it remained unclear whether government troops were in control of all or part of the approximately 14 square miles of debris fields.

Videos posted online appeared to show Ukrainian armored vehicles near the site, and reporters who visited earlier Sunday said insurgents were nowhere to be seen.