Sevilla have confirmed coach Jorge Sampaoli has departed to take charge of Argentina after compensation to trigger the release clause in his contract was received.
Sampaoli said: "The difficulty in the qualifiers is real".
At a press conference at the Argentinean Football Association (AFA) building in Buenos Aires on Thursday, Sampaoli said he had "100 players under observation", reports Xinhua news agency.
The 57-year-old, who is fulfilling a long-time dream of coaching his country's national team, also said he had signed a deal with the Argentinian FA until the 2022 World Cup in Qatar.
"I dream of building a strong squad".
But Sampaoli believes in their chances of reaching Russian Federation 2018, saying: "We are convinced that we have the resources to qualify".
The Argentinians' hopes of reaching the World Cup Finals next year are on a knife edge in the 10-nation South American group in which the top four qualify for Russian Federation.More news: Agassi stars as Djokovic romps to second-round win
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Argentina had been without a manager since they dismissed Edgardo Bauza on April 10 while holding the job for just eight months.
"I spoke with Leo and we're both excited", said Sampaoli.
Argentina will play Uruguay on August 31 in its next World Cup qualifying match.
"Messi? We want the best player in the world to feel happy here", he began.
Sampaoli has taken charge of the team at a time when their qualification for the 2018 World Cup to be held in Russian Federation is in jeopardy.
Sampaoli has top credentials, having coached Chile to a Copa America title in 2015 - beating Messi's Argentina in the final.