Messi has enjoyed a trophy-laden career at Barca, however, the five-time Ballon d'Or winner is still searching for his first major senior title with Argentina.
Coming off a Liga and Copa del Rey double, which took his trophy tally to 32 at Camp Nou, Messi is desperate to taste success with Jorge Sampaoli's men.
"Winning a title with Argentina would be something unique."
Argentina will face Iceland (June 16), Croatia (June 21) and Nigeria (June 26) in Group D at the World Cup, starting next month.
The two-time world champions – who last won the showpiece event in 1986 – scraped into the tournament thanks to Messi's magic.
"I have great faith in this group, we are working very well, we have players with a lot of capacity and experience," he said.
"But we do not have to throw the message of 'we are going to be the world champion because we are the best', because the reality is that it is not like that."
On Group D, Messi added: "Iceland are a team that showed in the last European Championship that they are difficult for anyone.