Sam Allardyce blamed West Ham's 2-1 Barclays Premier League defeat by Arsenal on an offside decision that left him "totally and utterly confused and bewildered".

Alex Song's 25-yard drive just six minutes into the London derby at Upton Park was disallowed due to three home players – among them Diafra Sakho – standing offside in front of Arsenal keeper Wojciech Szczesny.

The Gunners went on to score goals through Santi Cazorla and Danny Welbeck with Cheikhou Kouyate reducing the deficit, but Allardyce held referee Neil Swarbrick and his assistants responsible for denying his team at least a point.

"If we'd been allowed that goal we wouldn't have lost. We would have kicked on and Arsenal would have found life very difficult from then on," Allardyce said.

"I've not spoken to the ref because I don't see any point in confronting him when you know you're 100 per cent right and he hasn't had the visuals. You can't get anything out of telling him how wrong he's been.

"Going on what we were told at the start of the season, they got it horribly wrong. He (Sakho) wasn't in the eye-line of the keeper. He didn't touch the ball.

"Yes he was offside, but we've seen so many of those allowed as goals. I've not seen one 'not given'.

"Can you tell me what is an offside? Now I'm totally and utterly confused and bewildered. There are far too many inconsistencies."

Allardyce also disputed the first-half penalty that enabled Cazorla to open the scoring in an entertaining showdown in east London.

Cazorla tumbled over the body of Winston Reid as he darted into the area, but the West Ham defender seemed to lift a leg to help the Spaniard fall.

When asked if he thought it was a penalty, Allardyce said: "No. Reid stuck his leg up and he's gone over on it. I just wish Morgan Amalfitano had done something similar when he got tugged by Nacho Monreal in the last minute.

"He stayed on his feet. He was being fouled but didn't go down. There's another scenario for you – simulation is not nice, but when you don't get free-kicks for staying on your feet, you're in a difficult situation."

Arsenal's second successive 2-1 victory leaves them outside of the top four on goal difference alone ahead of the New Year's Day clash with Southampton at St Mary's.

"This game was more about fighting than being creative, but you need to have that in England, especially when you played two days ago," manager Arsene Wenger said.

"We gave so much physically that I was worried for this game. We knew before the game we would have to dig deep because we were jaded, but we did that.

"It's vital to re-establish ourselves in the top four. We have done it for 17 years so we are entitled to at least believe we can do it.

"But it's all to do because everybody fights very hard for it – Southampton, Tottenham, Man United. So many teams fight for these positions."