D.C. United have extended their lead at the top of the Eastern Conference with a smash-and-grab win at local rivals Philadelphia Union.

Chris Pontius struck the only goal of the game 12 minutes from time as he bundled home from close range following a low Branko Boskovic free kick.

The hosts had dominated the opening 78 minutes at PPL Park but Jack McInerney and Josue Martinez both fired good chances wide, while Freddy Adu and Michael Farfan were denied by desperate goalkeeping as United survived wave after wave of Philadelphia attacks.

Union responded well to going behind but, despite creating two more clear-cut chances, they could not find a way past Bill Hamid as the table-toppers recorded their third clean sheet on the road this season.