The Americans got two goals each from Samantha Mewis and Courtney Verloo in the win over the Ticas. Rachel Nuzzolese had a hat trick and Amber Brooks scored twice in the win over El Salvador, which was outshot, 42-0.
The USA faces Trinidad & Tobago on Tuesday. It will win Group A with a win or tie. T&T, coached by Notre Dame's Randy Waldrum, is second in Group with a 5-1 win over El Salvador and 0-0 tie with Costa RIca.
"We've played two games and we've gotten stronger from the first to the second," said U.S. U-17 coach Kazbek Tambi. "We'd like to have an excellent game against Trinidad (& Tobago) because we want to keep the momentum of the good soccer leading into the semis. At this point we're still taking it game by game."
Canada and Mexico have also booked a spot in the semifinals after winning their first two Group B matches.
The winners of Thursday's semifinals and the third-place game will qualify for the inaugural Under-17 Girls World Cup, which will be held from Oct. 28-Nov. 16 in New Zealand.
June 18 in Macoya
USA 6 Costa Rica 0. Goals: S.Mewis 3, 54, Verloo 17, 49, Roberts pen. 28, DiMartino 37.
USA -- Gaul, Brooks, Dunn, Colohan, Roberts, K.Mewis (Klei, 55), S.Mewis (Bennett, 57), Tymrak (Brian, 73), DiMartino, Johnson, Verloo.
Costa Rica -- Tapia, Barquero, Guillen, Cruz, Campos, Alvarado, Mata (Quiros, 46), Sanchez (Guzman, 66), Rodriguez Vazquez, Morales (Rodriguez, 19), Rodriguez Cedeno.
Referee: Dianne Ferreia James (Guyana).
June 18 in Macoya
USA 9 El Salvador 0. Goals: Nuzzolese 4, 64, 78, Klei 9, Brian 12, Costa 15, Brooks 27, 37, Bennett pen. 51.
USA -- Vancil, Dunn, Harris, Colohan, Brooks, Brian, Bennett, Klei, Payne, Costa, Nuzzolese.
El Salvador -- Rodriguez, Ramirez, Hernandez, Aldana, Duenas (Siguenza, 66), Flores, Landaverde, Herrera (Sanchez, 44), Gonzalez (Molina, 31), Fuentes, Ayala.
Referee: Carol Ann Chenard (Canada).