Robert Lewandowski struck twice as in-form Barcelona thrashed Red Star Belgrade 5-2 to stay in the upper echelons of the Champions League group table on Wednesday.

The veteran striker climbed to 99 career goals in the competition, while Inigo Martinez, Raphinha and Fermin Lopez also netted as Barcelona won comfortably in Serbia.

After thrashing Bayern Munich 4-1 in their prior European outing, Hansi Flick's side followed that up with a seventh straight victory across all competitions.

In that winning run Barcelona have scored at least three times in each game and were similarly lethal at the Rajko Mitic stadium.

"Obviously we are doing things very well, and many of them are being seen and reflected in our play, but there are certain things that I can tell you, as a defender, we're suffering in," said Barcelona centre-back Martinez.

"When the team drops (in focus) that little bit in the end... if we can improve that, I think we can be a great team and now we can enjoy this victory."

Fit again after injury, Frenkie de Jong started a match for the first time since April for Barcelona but Flick mostly selected his regular first choice side.

The German coach operates with an extremely high defensive line that has frustrated Bayern Munich and Real Madrid in recent weeks and after four minutes Red Star had a goal ruled out for offside.

Timi Elsnik broke in behind the defence and finished but had strayed beyond the last defender.

Barcelona took the lead after 13 minutes with Martinez heading home superbly from Raphinha's dangerous free-kick.

The Brazilian winger should have scored the second himself but lashed wildly over after a sensational ball from Lamine Yamal sent him racing in on goal, and also hit the post directly from a corner kick.

Red Star surprised La Liga leaders Barca by hitting back in the 27th minute when Silas got the better of the offside trap and clipped a finish past the onrushing Inaki Pena.

Gerard Martin, starting at left-back in place of Alejandro Balde in a rare rotation from the coach, was to blame for playing the Congolese forward onside.

Barcelona nudged their way in front shortly before the break when Lewandowski reacted quickest to finish the rebound after Raphinha's low drive hit the post.

The 36-year-old wasted a fine chance to add another when Raphinha teed him up but was able to bundle home his second from point blank range after 53 minutes when Jules Kounde's cross found him.

It was the Polish striker's 19th goal of the season in all competitions across 16 matches, and his fifth in four European games.

Only former Barcelona great Lionel Messi and Real Madrid icon Cristiano Ronaldo have scored more in the competition's history than Lewandowski, one short of a century.

Raphinha stroked home the fourth with a low effort from the edge of the box after more good work by Kounde.

Barcelona's 17-year-old centre-back Pau Cubarsi was forced off after sustaining a cut just below his eye from a high boot, on a rare negative note for the Catalans.

"He's laughing, he's got stitches, but he's young," said Flick after the game, indicating the defender was not seriously hurt.

Kounde completed a hat-trick of assists by cutting the ball back to substitute Fermin Lopez to slot the team's fifth goal past Marko Ilic.

Milson pulled another goal back for the hosts with a fine curling finish past Inaki Pena to offer slight consolation to the hosts.

Lopez hit the bar late on as Barcelona finished with five goals for the fourth time this season, and have scored a remarkable 55 in 16 matches across all competitions.

"We always can improve but what I have to say is I'm really happy," Flick told Movistar. "They played good, were good with the ball, and also we pressed them and it's good to see that."