WASHINGTON — An advocacy group representing American military veterans and Iraqi civilians arrived here on Wednesday armed with a message for the United States government: Washington must do something for the thousands of people suffering from what the group called the “environmental poisoning” of Iraq during the war.

The group, Right to Heal, says that veterans and civilians continue to feel the effects of the burn pits — banned by Congress four years ago — that were used to dispose of military waste, and that new health problems arise every day for Iraqis.

“Things are worse off today by a thousandfold,” Representative Jim McDermott, Democrat of Washington, said during a hearing in the House on Wednesday morning that featured witnesses from Right to Heal.

Several hours later, Right to Heal called its own “people’s hearing” at a Quaker meeting house in Washington. One witness there, John Tirman, executive director and principal research scientist at the M.I.T. Center for International Studies, said that in playing down the health effects of the war, American officials had violated “the trust we place in government, that is, that they would be accountable to us even in the most severe times of war.”