Gunners shoot down the Hammers
Arsenal moved above West Ham to the Premier League’s fifth position after inflicting a first home defeat on the Hammers since August.
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Alex Song had a 20-yard volley ruled out for offside for the hosts.
Santi Cazorla put the visitors ahead from the penalty spot, following a foul by Winston Reid, before Danny Welbeck put Arsenal further in front.
Cheikhou Kouyate's header gave the hosts hope but West Ham were unable to avoid a second successive defeat.
It was Arsenal's ninth win in a row in all competitions against West Ham as they moved level on points with fourth-placed Southampton and just three points behind Manchester United in third.
Former Arsenal man Song, on loan from Barcelona, almost made an instant impact with a low volley into the bottom corner of the net from 20 yards early on. Sakho was penalised for standing in an offside position in front of goal, though, despite not touching the ball.
Cazorla punished Reid's high leg in the box to roll home the resulting penalty for the opener. The lead was doubled minutes later as Welbeck was on hand to slide the ball into the net from close range after Alex Oxlade-Chamberlain's low cross from the right.
Stunned by the deficit, West Ham restored hope soon after the break with Kouyate bundling Sakho's cross from the right into the net via Mathieu Debuchy.
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But it was Arsenal who had the better chances, with Hammers keeper Adrian brilliantly denying Alexis Sanchez and Oxlade-Chamberlain, before the visitors survived a late aerial bombardment.