The US Supreme Court has agreed to consider whether vitriolic anti-gay protesters who picket the funerals of US soldiers are protected by free speech laws.

The emotionally-charged case was brought by the family of US Marine Matthew Snyder, who was killed in combat in Iraq in 2006.

His family organized a private Christian funeral for him in Maryland that attracted members of the radical Westboro Church led by Baptist preacher Fred Phelps.

Phelps and his congregation regularly demonstrate at military funerals, carrying inflammatory signs to draw attention to their anti-gay message.

The religious group protest at the funerals of soldiers, regardless of the sexuality of the deceased military personnel, and use the events to bring publicity to their campaign.

The preacher and six relatives arrived at Snyder's funeral carrying signs that read "America is doomed," "Matt in hell" and "Semper Fi fags," in reference to the Marine motto "Semper Fi."

After the funeral was over, Phelps continued to deride and criticize Snyder on his website, prompting the dead Marine's family to sue the preacher before a Maryland court.

Snyder's father Albert claimed Phelps had intruded on a private event and intentionally inflicted emotional distress on the bereaved family and won an initial award of five million dollars.

But the award was overturned on appeal, where a court ruled that Westburo protesters were simply exercising their First Amendment right to free speech.

Protests launched by Phelps and his congregants have been met with revulsion across the United States and around 40 states have now passed laws regulating demonstrations at funerals.