Christians living in areas controlled by the Islamic State continue to be killed en masse by the radical Muslim group, threatening their existence in the region, according to a bipartisan delegation of lawmakers who are calling on the Trump administration to immediately take steps to end what they describe as genocide.

Not a single ISIS perpetrator has been held accountable for genocide, crimes against humanity, or war crimes since former Secretary of State John Kerry formally declared genocide was taking place in the region, according to the group of lawmakers, who slam the former administration for its lack of action.

The United Nations has also failed to take action, prompting accusations that the international organization is overlooking the systematic slaughter of Christian populations who have lived in the region for centuries.

The lawmakers are seeking to ensure that the Trump administration does not ignore the issue, as the Obama administration did for years while ISIS slaughtered Christians and other minority groups.

"One year since the genocide declaration of former Secretary Kerry, ISIS continues to terrorize innocent civilians in Iraq, Syria, and all over the world," according to a letter written by Rep. Sean Duffy (R., Wis.) and supported by Reps. Chris Smith (R., N.J.), Anna Eshoo (D., Calif.), Jeff Fortenberry (R., Neb.), and Sen. Bill Cassidy (R., La.). "To date, no ISIS perpetrator has been tried for genocide, crimes against humanity, or war crimes, evidence that the international community has done too little to hold them accountable."

The letter was sent on Friday to Secretary of State Rex Tillerson and U.S. Ambassador to the United Nations Nikki Haley.

The lawmakers ask the Trump administration to provide an update on the specific actions the United States has taken since Kerry's genocide declaration.

Christian populations in ISIS-controlled territory have significantly plummeted, according to State Department figures provided by the lawmakers.

In Iraq, at least 250,000 remain, down from an estimated 1.4 million in 2002, and 500,000 in 2013, the year before ISIS began mass killings of minorities.

In Syria, Christians were estimated to constitute 8 to 10 percent of the country's 21 million population. Since ISIS took control, that number is "considerably lower," according to the lawmakers.

"They are entitled to justice as much as the other religious and ethnic minority groups that ISIS targeted for destruction," the lawmakers write.

As with the former Obama administration, the U.N. also has "overlooked Christian victims of this genocide and the effects of this crime: Christians are at-risk of being wiped away from the lands where they have lived since the first century," according to the letter.

"Key entities" within the U.N. have repeatedly ignored the issue, causing concerns that some are seeking to sweep it under the rug.

The United States must step up efforts to pressure the U.N. into taking action, the lawmakers say. This would include formal investigations and future trials of those ISIS members found to be orchestrating mass killings.

"We urge you to ensure that the United States is leading efforts at the United Nations and elsewhere to bring worldwide attention to the atrocities committed by ISIS against religious and ethnic minorities and to bring the perpetrators to justice," the lawmakers write. "We urge you to press the U.N. Security Council to authorize a formal investigation into genocide and other atrocity crimes of ISIS, against the full range of victims, so that the perpetrators can be brought to justice."

The Trump administration will have the support of both Democrats and Republicans in Congress as it moves to "ensure that ISIS perpetrators are investigated, tried, and convicted for the genocide, crimes against humanity, and war crimes they have committed," the lawmakers write.