MINNEAPOLIS -- The Minnesota Vikings played their coldest home game in 38 years on Sunday, when they beat the Carolina Panthers in 12-degree temperatures at TCF Bank Stadium.

As both teams dealt with the freezing temperatures, Fox cameras showed sideline attendants using heaters to warm up game balls, which is against league rules. NFL vice president of officiating Dean Blandino said Monday morning on NFL Network that officials warned both the Vikings and Panthers not to heat up the balls during Sunday's game, and would remind teams this week not to heat game balls.

"You can't do anything with the footballs in terms of any artificial, whether you're heating them up, whether it's a regular game ball or kicking ball, you can't do anything to the football," Blandino said. "So that was noticed during the game, both teams were made aware of it during the game and we will certainly remind the clubs as we get into more cold weather games that you can't do anything with the football in terms of heating them up with those sideline heaters."

By rule, the home team is responsible for providing the game balls. The footage shown on NFL Network is from the Vikings' sideline, though there are attendants from both teams on both sidelines and a league source said the Vikings weren't heating up the balls during the game.

Zimmer said he hadn't heard anything from the league about it on Monday, adding, "Somebody told me (Carolina's) ball boys were doing it."

Ball boys are allowed to carry hand heaters, as Fox officiating analyst Mike Pereira pointed out, but they are not allowed to put game balls in front of sideline heaters. The Vikings have two December home games on their schedule -- this Sunday against the New York Jets and Dec. 28 against the Chicago Bears. After the league sent out a warning, it stands to reason they'll be watching the sidelines closely in cold-weather games this month.