With their Champions League round-of-16 tie just over a month away, things are already heating up between Manchester United and Sevilla.

Sevilla have already faced United's bitter rivals Liverpool this season, finishing second behind them in the group stages after two entertaining draws.

They drew 2-2 at Anfield in the opening round of fixtures, and then miraculously recovered from 3-0 down at half-time at the Ramón Sánchez Pizjuán Stadium to keep their hopes of qualification alive.

United's passage to the knockout stages for the first time since 2014 was much more straightforward, securing five wins out of a possible six, though a 1-0 defeat in Basel almost put their spot in jeopardy.

With Mourinho in charge, they will feel well-equipped to attack the latter stages of the competition, with the Portuguese a winner of it twice with Porto and Inter Milan.

But the clash between the two sides scheduled for next month will be a fiery one, with Sevilla charging travelling United fans £89 per ticket - £35 more than Liverpool's supporters were.

It sparked fury from the Manchester club, and after negotiations to lower the price failed to reach a compromise, they revealed they were upping Sevilla followers' tickets to the same total for the return leg at Old Trafford.

They would refund £35 to each United fan who attends the first leg, then send any extra profit they make from the increased charges to the Manchester United foundation.

It appeared that would be the end of the matter, but Los Rojiblancos have hit back by threatening to report United to UEFA.

They claim the Premier League side have given them almost a per cent less than the mandatory allocation for visiting fans for the return leg.

Sevilla are also, ironically, unhappy that United have bumped the prices up in retaliation to their own actions, and want the European governing body to intervene.

"Right now everything is hypothetical as United have not informed the club officially of the price rise," a club source told Reuters, per the Mirror.

"Sevilla will demand through UEFA that United comply to the ticketing rules that tickets for home and away fans have a similar price for similar categories.

"Sevilla will inform UEFA that United have not complied with the rule that away teams are offered 5 per cent of the capacity in tickets, only offering 4.1 per cent."

It doesn't seem as though this feud is going to end any time soon, with the first leg on February 21 fast approaching.