Birmingham 0 Arsenal 3: Arsenal kept the pressure on the two Manchester clubs at the top of the Barclays Premier League as they overwhelmed Birmingham with a 3-0 win at rainswept St Andrew's.
A Robin van Persie free-kick and Samir Nasri's 13th goal of the season put the visitors in control before an own goal from Roger Johnson added to City's misery.
The Gunners remain in third spot, two points adrift of United and City, after a sixth away success of the season.
Arsenal started brightly and Theo Walcott skated past the challenge of left-back Liam Ridgewell but shot into the side netting from a narrow angle.
Arsene Wenger's side were in the ascendancy and after 13 minutes Van Persie put them ahead when Scott Dann was adjudged to have brought down the Dutchman down. Van Persie's free-kick took a deflection off Lee Bowyer and flew past Ben Foster into the corner of the net.
Birmingham tried to respond and it needed a fine save from Lukasz Fabianski after Cesc Fabregas had fouled Craig Gardner.
Arsenal were carving out plenty of chances and Van Persie found himself with only Foster to beat, but he tried to clip the ball over the Blues keeper and failed to make proper contact on his shot.
Foster then did well to block an effort from Nasri after a mistake by Johnson had allowed him a free run at goal, but it came as no surprise when Nasri made it 2-0 after 57 minutes.
The France midfielder exchanged passes with Fabregas before driving a low shot past Foster just inside the box.
Eight minutes later it became 3-0 with an own goal from Johnson. Nasri set up Fabregas, whose shot was blocked by Foster, but the loose ball bounced off Dann and then Johnson and over the line.