Clint Dempsey's bout of ref rage could see the USA captain miss the Gold Cup, if US Soccer determine his ripping up of a referee's notebook warrants a three-month ban.

Dempsey was sent off after snatching referee Daniel Radford's notebook and tearing it up in extra time of his Seattle Sounders' 3-1 loss to bitter regional rivals Portland in the fourth round of the US Open Cup on Tuesday.

He had already received a yellow card for dissent, and lost his cool when team-mate Michael Azira was shown a red card for a challenge on the Timbers' Gaston Fernandez in the 112th minute.

The outburst from former Fulham and Tottenham man Dempsey was just the most spectacular of a more than testy match, after which the officials were escorted from the field by police.

The Sounders finished the game with seven men, with Brad Evans also sent off in the 69th minute and Obafemi Martins stretchered off after all three substitutions had been made.

US Soccer, rather than Major League Soccer, oversees the US Open Cup, and Sports Illustrated's Grant Wahl reported Wednesday that the federation was gathering information on the incident, which could be found to be "referee assault".

According to the federation's policy manual, referee assault includes "damaging the referee's uniform or personal property" and carries a suspension that could range from six matches to three months.

The Dempsey drama comes as the United States prepare to defend their title in the CONCACAF Gold Cup which starts on July 7.

The winners of the regional championship for North and Central America and the Caribbean book a place in the 2017 FIFA Confederations Cup, and the Americans expected to go into their title defence on a high after recent friendly victories over the Netherlands and Germany.