Laurent Koscielny admits Arsenal failed to "stay together" as they were humbled by Bayern Munich on Wednesday night - but refused to throw the towel in on the Gunners' Champions League campaign.

Arsene Wenger's side were thrashed 5-1 at the Allianz Arena in the opening leg of their round-of-16 clash with the Bundesliga champions, with four of the Bayern goals coming after Koscielny had been forced off through injury.

The match was level when the Frenchman limped off four minutes into the second-half after Arjen Robben's strike had been cancelled out by Alexis Sanchez, who tucked away the rebound from his own saved penalty.

But the hosts turned the screw and struck three goals in 10 minutes courtesy of a Robert Lewandowski header and a brace from Thiago - with substitute Thomas Muller adding the gloss late on.

Not only does the result leave Arsenal staring at a seventh straight exit from the Champions League at this stage of the tournament, it has also seen a clamour for Wenger to leave when his contract expires in the summer.

The Frenchman has repeatedly said throughout the season he will not make a decision on his future until then and that has not changed, even though some want to see him go in the immediate aftermath of Wednesday's defeat.

Wenger said the game was a "nightmare" for his side and Koscielny's assessment was similarly damning.

Asked what went wrong, Koscielny told Arsenal TV: "I don't know either. I think the first half was good - we were deep, like you need to be, and tried not to give them space or opportunities to score.

"It being 1-1 at half-time was good for us but after we conceded the second goal, I think we needed to stay more compact because we have a second leg at home, when you can win.

"You need to stay together, defend together with all 11 players and keep this. Tonight we didn't. Bayern played very well - they have some top-class players who can make the difference up front."

Despite the second-half capitulation, Koscielny insists Arsenal will fight to overturn the chasm in the home leg, but they would have to become the first team in the history of the Champions League to come back from a four-goal deficit and advance.

"In football, you never know," he added.

"We are professional, we have a second leg and we will try to win. But after a game like this it's very difficult. But we'll fight because we are professional and we play for Arsenal."

Bayern boss Carlo Ancelotti was also careful not to suggest his side had already booked their place in the quarter-finals.

The five-time winners blew their visitors away during a scintillating second-half display, with Philipp Lahm - playing his first Champions League game since announcing his intention to retire in the summer - suitably impressed.

"We played very well before the penalty, then not so well until half-time, but sensationally after the interval," he said. "We demonstrated our qualities in possession and scored the goals at the right time. The 5-1 victory puts us in a very, very good position for the return."

While Bayern celebrated another massive win, Wenger's future was being questioned by a host of former Arsenal favourites.

Former full-back Lee Dixon believes Wenger's demeanour suggested he was considering his options as he told ITV Sport: "He just seems so low. I think he's realising that with this team he's getting no response from them. They're not doing themselves justice or him.

''That is the first time where I've seen him where I've thought, 'he thinks it's time'. The fact that he hasn't been able to get a response from the players in the last few weeks might be the final straw.''

Martin Keown, who won three league titles under Wenger, suggested this was the nadir of the Frenchman's tenure.

''It's almost embarrassing - outclassed, outplayed,'' he told BT Sport. ''This is his lowest point ever. Twenty years and Arsene must be considering his future now. Arsenal were bullied, weren't they? Completely.

''The difference between the two squads, they (Bayern) were men against boys, they were bigger and stronger.''

Meanwhile Ian Wright, who claimed last week that Wenger had told him he was growing ''tired'', was tweeting throughout the match and showed his displeasure at the performance.

''We are a ******g shambles,'' he posted. ''At least let it go to the ******g home leg.

''**** it !!! Not watching anymore.''

And Bob Wilson, the former Arsenal goalkeeper who also worked as a coach at the club during Wenger's early years, labelled the result a "horror story" and asked whether the night could prove a tipping point.

"The big question now is, is this going to tip Arsene over the edge?," he told BBC Radio 5 Live. "He's a hugely intelligent man; he can only take so much.

"He lives for the club, totally dedicated, very hurt when things go wrong. But as everybody knows and everybody sees now, his longevity in the job, 20 years, seems to be a little bit like a millstone round his neck when things go wrong."