Lionel Messi said Argentina's shock loss to Saudi Arabia was a "very heavy blow" but vowed the South American champions would bounce back from the defeat.

"It's a very heavy blow, a defeat that hurts, but we must continue to have confidence in ourselves," Messi said after Argentina crashed to a stunning 2-1 loss in Group C.

"This group is not going to give up. We will try to beat Mexico," he added.

Messi gave Argentina a 10th-minute lead from the penalty spot on Tuesday at the Lusail stadium but second-half goals from Saleh Al-Shehri and Salem Al-Dawsari helped the Saudis pull off one of the greatest shocks in World Cup history.

It could have been so different but for three tight offside calls in the first half that went against Argentina, denying them three potential goals.

In the second half, coach Lionel Scaloni's men failed to exploit the Saudis' high defensive line in the way they had managed before the break.

"We know that Saudi Arabia is a team with good players, who move the ball around well and play a high line," said Messi.

"We worked on that but we rushed it a bit. There are no excuses, we're going to be more united than ever before.

"This group is strong and has shown it before. We had not been in this situation for a long time. Now we must show that this is a true group."

The Albiceleste have two Group C games remaining -- against Mexico on Saturday and then Poland the following Wednesday -- to try to salvage their hopes of claiming a third world title.

Playing in his fifth and probably final World Cup, Messi has the weight of an entire nation's expectations on his shoulders.

One of the greatest players of all time, this may be his last chance to lift football's greatest prize, which his predecessor as Argentina's idol Diego Maradona did in 1986.

"We have to go back to the basics of who we are. We have to think about what's coming next," said the seven-time Ballon d'Or winner.