US Secretary of State John Kerry on Thursday declared that violence by Islamic State against religious minorities was genocide, saying the United States would do everything it could to hold the militant group accountable.

"In my judgment Daesh is responsible for genocide against groups ... under its control, including Yazidis, Christians and Shia (Shi'ite) Muslims," Kerry said in a statement to reporters, using another term for Islamic State.

"Naming these crimes is important, but what is essential is to stop them," Kerry said. 

A genocide determination is not likely to have any practical impact on US policy against the militant group by President Barack Obama, who has bombed the militants but has sought to avoid any large-scale commitment of US troops.

On Wednesday, a State Department spokesman Mark Toner said that "acknowledging that genocide or crimes against humanity have taken place in another country would not necessarily result in any particular legal obligation for the United States."

Islamic State militants have swept through Iraq and Syria in recent years, seizing control of large swathes of territory with an eye toward establishing jihadism in the heart of the Arab world.

Its videos depict violent deaths of people who stand in its way. Opponents have been beheaded, shot dead, blown up with fuses attached to their necks and drowned in cages lowered into swimming pools, with underwater cameras capturing their agony.

US lawmakers urged Kerry last year to make a determination on whether atrocities committed by the militant group against Christians and other religious groups amounted to genocide.

The State Department earlier had predicted it would miss a deadline set for Thursday on this decision.

Republicans, who control the US Congress, have been pressuring the Democratic Obama administration to call the militants' atrocities in Iraq and Syria acts of genocide. This week, one chamber, House of Representatives, passed a resolution labeling the group's violence against religious and ethnic minorities as genocide, war crimes and crimes against humanity.

A US administration has not made a genocide finding since 2004, when it determined genocide had occurred in Darfur and that the government of Sudan was responsible.