Portugal manager Paulo Bento has admitted that Northern Ireland have the potential to cause an upset when the two sides meet in Belfast.

Michael O'Neill's side are the underdogs on paper, but pulled off a surprise 1-1 draw when the teams last met in their World Cup qualifying group.

"I expect Northern Ireland to play with a similar structure to what we saw in Porto... they are aggressive and play direct football," Sky Sports News quotes him as saying.

"They have got quality and maybe I expect them to take on the match a bit more than last time. Northern Ireland have a chance, so maybe it is possible that they attack a bit more."

Northern Ireland also defeated Russia 1-0 last month, a result which Bento believes should serve as a warning to his players.

"The Russia match was another warning. We knew this match would be tough but that game and the way they beat Russia showed that they're a really strong squad. It was a warning for us," he added.

Northern Ireland take on Portugal tomorrow evening at Windsor Park.