Two first-half goals helped D.C. United to a 2-0 win over Arabe Unido and a spot in the knockout stage of the CONCACAF Champions League.

Conor Doyle and Jared Jeffrey were both on target at RFK Stadium as United preserved its perfect record and became the first team to reach the quarter-finals by winning Group H with a game to spare.

The win saw D.C. — yet to concede a goal after three matches in the continental competition — eliminate Unido and Montego Bay United, with three and zero points respectively.

United wasted no time asserting its dominance over the Panamanian visitor thanks to Doyle, who opened the scoring less than a minute into proceedings.

Doyle fired a shot low and hard across Unido goalkeeper Miguel Lloyd with barely 40 seconds on the clock after Chris Pontius's cross found his teammate at the back post.

Jeffrey then doubled the lead in the 22nd minute, heading a corner into the ground and past Lloyd.

Unido hit the crossbar twice early in the second half, with Renan Addles and Abdiel Macea rattling the woodwork, but victory for United was never in doubt as the MLS side progressed to the last eight for a second straight season.

In Wednesday's (AEST) only other match, Santa Tecla and Real Salt Lake played out a scoreless draw in El Salvador.

RSL had midfielder John Stertzer sent off for a second yellow card with 10 minutes remaining, though the MLS club held on for a point.

Municipal and RSL, which has a game in hand, are level on four points in Group G, two clear of Santa Tecla.