Manchester United have made enquiries about Tottenham defender Toby Alderweireld and Shakhtar Donetsk midfielder Fred.
The two players would cost United a combined fee in excess of £100m as manager Jose Mourinho seeks to strengthen the spine of his team.
United hope to complete a deal for Brazilian Fred before the World Cup.
Tottenham have an incentive to cash in on Alderweireld as he will be able to leave for £25m in 12 months' time.
United defender Matteo Darmian, meanwhile, is interesting Juventus.
The Italian, 28, has a year left on his contract and started only five Premier League games for United during the season just completed.
Along with Daley Blind and Luke Shaw, Darmian has little future at the club as Mourinho, who signed a new contract in January that will keep him at Old Trafford until at least 2020, seeks reinforcements.
United finished second in the Premier League - their highest placing since Sir Alex Ferguson retired in 2013 - but were 19 points behind champions Manchester City.
Belgium defender Alderweireld and Brazil midfielder Fred have both been selected for their World Cup squads.
However, in Fred's case, United could take advantage of Brazil's presence in England next week to complete a medical should the transfer progress positively.
Brazil will train at Tottenham after arriving in England on Sunday before their friendly with Croatia at Anfield on 3 June.
Alderweireld, 29, was involved in just six Tottenham games after declaring he had recovered from a hamstring injury in January, having failed to force his way back into the team.
Fred, 25, has been heavily linked with Manchester City, but they have since turned their attentions to Napoli midfielder Jorginho, leaving the way clear for United to negotiate with Shakhtar.
Rinat Akhmetov, Shakhtar's president, has confirmed talks with United are ongoing and has indicated that Fred will cost more than the £34m that Manchester City paid for Fernandinho in 2013, and the reported £43.8m that allowed midfielder Alex Teixeira to join Chinese club Jiangsu Suning in 2016.
"We will sell Fred to a good and strong club, do not doubt it," Akhmetov told the Great Football program on Ukraine's Football 1.
"He will cost the potential buyer more than Fernandinho and be even more expensive than Alex Teixeira."
Mourinho still hopes midfielder Marouane Fellaini will sign a new contract, although the Belgian has yet to make a final decision and hopes are receding at Old Trafford that he will stay now that he can leave for nothing.