Argentina captain Lionel Messi insisted his side will improve after sealing their place in the last 16 of the World Cup with a 1-0 win over Iran on Saturday.

The four-time World Player of the Year curled home his second goal of the competition to snatch all three points after Argentina goalkeeper Sergio Romero had kept Iran at bay with three fine second-half saves.

"We are not showing what we can do, but with a run of games I think we will get there," said the Barcelona forward.

"We are the first ones to know we have not reached the level we would like. Despite that, though, we had a lot of chances in the first-half."

With Gonzalo Higuain restored to the starting line-up alongside Messi and Sergio Aguero, Argentina were expected to have far too much firepower for Iran.

However, Messi admitted that the toil of playing in the early afternoon heat in Belo Horizonte had contributed to his side's difficulty in breaking down a well-organised Iranian defence.

"Today it was very difficult for us. They didn't leave any space behind and, moreover, we suffered a lot with the heat.

"In the first-half we had a lot of chances and in the second they came out a bit more and had theirs. Luckily we managed to win at the end and seal qualification to the next round.

"When the ball went in I felt very happy to have qualified because it was in the last few minutes and for the roar of the crowd. It was a lovely moment."

Manchester City defender Pablo Zabaleta confessed he couldn't see his side making the breakthrough as Iran had looked the more likely team to score in the latter stages, but claimed anything is possible with Messi on your side.

"We haven't seen the best version of this team, I think there is more to come," he said.

"We had the feeling that we weren't going to win, but when you have a player like Leo, who can always do that, it is very satisfying."

Iran boss Carlos Queiroz agreed that Messi had been the difference between the two sides.

"Messi was always there insisting and insisting, always looking for space. He is the best in the world.

"The game went as I expected. We have faced a fantastic Argentine team that has controlled the ball and we were very disciplined and concentrated. We saw a very intense game high on emotion."

However, Queiroz was also furious at referee Milorad Mazic for not awarding a penalty when Ashkan Dejagah clashed with Zabaleta with the score at 0-0 in the second-half.

"Messi was fantastic, but the referee wasn't," said the former Real Madrid coach.

"I don't understand it, he had to have seen it was a penalty. He was five metres away, there is no way he couldn't have seen it.

"I have the right to say this and I only hope I am now not punished for telling the truth.

"The referee hasn't done what he had to. It was a clear penalty. In a game like this, even more so for a team like Iran, these details make the difference."