Mesut Ozil opened the scoring with Arsenal's first shot on target after Angelo Ogbonna's mistake and Sanchez's cut-back set up a simple finish.

The Hammers threatened to equalise after the break before Sanchez turned, drove into the box and finished brilliantly past Darren Randolph.

The Chilean added a second eight minutes later with a low strike into the corner, before Andy Carroll came on for a first appearance since 18 August and stooped to nod in the rebound from a Dimitri Payet free-kick that hit the bar.

However, any faint hopes of a comeback were extinguished by Alex Oxlade-Chamberlain's excellent finish from range and Sanchez's masterful dinked chip over the keeper.

The heavy defeat leaves Slaven Bilic's side one point above the relegation zone after Sunderland beat Leicester 2-1 earlier on Saturday.

Arsenal trail leaders Chelsea by three points but will slip to third if Liverpool win at Bournemouth on Sunday.

Arsene Wenger's side are now unbeaten in 13 league games, and their last away defeat was in February.