The video assistant referees were called upon for several key decisions at the Confederations Cup in Russia on Sunday, as Portugal were held 2-2 by Mexico and Chile beat Cameroon 2-0.

The first senior global tournament to have video technology for decisions other than goalline calls saw it come into focus on the second day of action.

"There are immediate feelings that have to do with the emotional part that surrounds football and to reverse that will take some time and get used to it," was Chile coach Juan Antonio Pizzi's view on the VAR system.

A dramatic Group A game in Kazan that saw Hector Moreno head in an injury-time winner for Mexico also featured a goal by Nani disallowed for offside on review.

Chile left it late to edge out a dogged Cameroon in an opening Group B clash that saw striker Eduardo Vargas have goals both ruled out and given by the video assistant.

Portugal captain Cristiano Ronaldo went into the day's first match with plenty of attention on his future, after reports that he wanted to leave Real Madrid after being charged with tax fraud.

The forward teed up Ricardo Quaresma to put the European champions ahead, but Javier Hernandez levelled on the stroke of half-time.

Before that though Fernando Santos' Portugal had been denied the opener after Nani converted from a rebound when Ronaldo's volley had rattled the crossbar.

But the referee consulted with the video official, who noticed that four Portuguese players had been offside in the passage of play before Ronaldo's shot.

Southampton full-back Cedric Soares restored the Portugal advantage with four minutes left when his drilled effort looped in off Moreno.

But the Mexican defender climbed highest to power home a header from a corner in added time and snatch a point.

- Vargas sees both sides of video ref -

Chile striker Vargas missed a whole host of chances in his side's victory over Cameroon, but became the first player to ever have goals disallowed and awarded by a video official in the same match.

The former Napoli man had an eventful day to say the least, as he struck the post in the first minute, saw a shot saved and blazed an excellent chance over before he eventually found the net in first-half stoppage-time.

Over a minute had passed since Vargas' exuberant celebrations when the on-field referee signalled the goal had been chalked off.

Initial replays appeared to show the forward level with the last defender, only for the technology that draws a line across the pitch to reveal half of his head was in an offside position.

Confusion reigned in the stadium while Vargas and teammate Arturo Vidal frantically pointed at the replays on the big screen to try and prove their point as the players walked off for the interval.

The Copa America champions were by far the better side over the course of the encounter, and deservedly broke the deadlock through Vidal's excellent 81st-minute header.

Alexis Sanchez, brought on as a second-half substitute after picking up an ankle injury in training, ran clean through on goal late on with a national record 38th Chile goal for the taking.

His shot was blocked, but with the Cameroon goalkeeper Fabrice Ondoa stranded, Vargas tapped in.

The 27-year-old was flabbergasted to see the linesman's flag raised, but video replays showed that Sanchez was onside in the build-up and Vargas' 34th international goal was given.

The 2018 World Cup warm-up event continues on Monday, as Germany face Australia in the last of the opening round of fixtures.