Coach Tony Popovic will travel to Sarasota on Wednesday, along with a group of players who have already completed their domestic season.
Popovic will use the Sarasota camp to assess an extended number of players in a train-on squad, before finalising his selection on June 1.
Souttar, Leckie, Mitch Duke, Hayden Matthews, Anthony Caceres, Brandon Borrello, Nick D'Agostino and Nishan Velupillay will all head into camp.
Popovic clarified on Monday these were all players he wanted to look at that either had limited recent game time, or their seasons had ended more than a week ago.
Souttar has played two games for Leicester since returning from a ruptured Achilles, then knee surgery, and has looked commanding as ever.
Melbourne City veteran Leckie has never played under Popovic, previously withdrawing from multiple squads due to soft-tissue injuries.
The 35-year-old made a late-season return from hip surgery then turned in an extraordinary 120-minute performance before scoring a penalty as City lost their elimination final to Auckland in a shootout.
Popovic said Leckie's performance showed the difference between emerging youngsters and experienced international players like Qatar 2022 heroes Leckie and Souttar.
"Mathew Leckie was the best player on the park at 35 and no football under his belt - that a young player can't do as yet. That's the difference, and that's what you need at a World Cup," Popovic said.
Popovic added: "Both those cases, Harry and Mathew, if you see how long they've been out for, that's a mental resilience that you can't buy.
"You have to go through that adversity, and they've done that.
"... If you watch him (Souttar) perform, and you watch those two games, there is presence, there is aura, there is a leader. You can't get that in a young player overnight. That takes time.
"And he's done it at a World Cup when he played also underdone.Â
"So they've shown the resilience and mentality that you need at a World Cup, because you need to be able to suffer. You need to be able to go through tough times."
Popovic was delighted with Leckie's performance after just 49 minutes in his three preceding games since returning, saying it showed his strength.
"These two players just illustrate the World Cup is on another level again to anything we've done so far," he said.
City phenomenon Marcus Younis wasn't among those named but Popovic said the door wasn't closed yet for the winger and other players.
More players will enter camp across the coming weeks, while the Socceroos will keep monitoring Craig Goodwin after his influential return for Adelaide.
Popovic hoped to receive an update on midfielder Aiden O'Neill's ankle injury within the next 24-48 hours.
After being stepped on, O'Neill went down twice then came off the field just 36 minutes into New York City FC's 2-0 loss to DC United on Sunday (Monday AEST).
O'Neill, looking to play in his first World Cup finals, has started Australia's past seven matches and been Popovic's first-choice defensive midfielder along with Jackson Irvine.
O'Neill's NYCFC coach Pascal Jensen confirmed he had already been playing with an ankle problem but his ankle had "locked and had too much pain to continue".
The Socceroos play a friendly against Mexico in Los Angeles on May 31 - the last game before Popovic selects his squad on June 1 - before relocating to their Bay Area base.
Australia then play Switzerland on June 7 in San Diego, their final warm-up game.
The Socceroos face Turkey in Vancouver on June 14, the United States in Seattle six days later then Paraguay in Santa Clara on June 26 (all dates AEST).
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