Belgium need to improve on their performance in Monday's 3-0 win over Panama as their World Cup campaign progresses against Tunisia, according to head coach Roberto Martinez, but he said they "should not look for perfection".

After a laboured first-half against the unfancied Central Americans, Belgium took the lead through Dries Mertens' superb volley before hitting their stride and adding further goals from Manchester United striker Romelu Lukaku.

Martinez acknowledged that his side's slow start had attracted criticism but he said the game ultimately lived up to his hopes.

He told reporters: "If you had told me before the game, you're going to win the game by a three-goal margin, you're going to score three goals from open play, beautiful goals, because the hardest thing in football is to score goals and they were beautiful goals from open play, you're going to keep a clean sheet, I would have said this, this is exactly what we wanted.

"You should not look for perfection. We need to improve of course. We want to improve, we need to improve. I think the three games in the group stages, it was always for us to improve in a mental way, in a technical way, in a tactical way.

"I think physically we were very strong, we grew into the game in the second half, we felt physically very strong - which is a great sign."

Nabil Maaloul's Tunisia were holding England to a 1-1 draw in their opening match until Harry Kane scored the winning goal in second-half stoppage time of their Group G encounter.

And Martinez said he was impressed by what he saw from Gareth Southgate's side.

"Everyone has got an opinion and everyone is entitled to an opinion. We have watched their game and to be fair, it was good to see the spirit that they have," he said. "They scored two goals from a corner, on the 91st minute to win the game.

"We just are concentrating on ourselves. We need to make sure that we are as good as we can be against Tunisia and then against England."