A US Congressional letter to the Prime Minister expressing concern about the UK Government’s "dangerous" Troubles legacy plans is a "significant intervention", Michelle O’Neill has said.
The bipartisan letter from 27 members of the US Congress claims draft legislation going through Parliament would "deny justice to thousands of families" and "conceal the truth of the past".
The Sinn Fein vice president said the letter was further evidence of "widespread and consistent" international opposition to the Government's contentious proposals.
The Northern Ireland Troubles (Legacy and Reconciliation) Bill would provide immunity for people accused of Troubles offences, as long as they co-operate with a new truth recovery body, and would also halt future civil cases and inquests linked to killings during the conflict.
The Government has proposed several amendments to the Bill as it proceeds through its legislative stages, though the core elements of it remain.
Critics have denounced the proposed laws as offering an "amnesty" for killers and the Bill is opposed by all Stormont's main parties and the Irish Government, as well as victims' groups in Northern Ireland.
Signatories to the letter, which has been seen by the PA news agency, include Congressmen Brendan Boyle and Richie Neal.
The letter from the Congress members highlighted their strong support for the UK and US's "special relationship", and congratulated Rishi Sunak on becoming Prime Minister.
It added: "As you assume this critical role at a precarious point in Europe's history, we write with grave concern over your Government's decision to continue advancing the Northern Ireland Troubles (Legacy and Reconciliation) Bill which denies justice, suppresses the will of the people of Northern Ireland, and conceals the truth of the past.
"Amid this time of war and unrest on the continent, we are concerned that this legislation would undermine the Belfast/Good Friday Agreement, the bedrock of peace in Northern Ireland.
"As Members of Congress who understand and respect the pivotal role that the United States played in coming to this Agreement, we feel compelled to write you to voice our strong concern and disappointment with the continued momentum this piece of dangerous legislation maintains.
"We believe this legislation would deny justice to thousands of families, across all communities in Northern Ireland, who were impacted by violence during the Troubles.
"We have conducted significant research into the matter, and we have listened to victims, victims' relatives, human rights groups, political representatives in Ireland and Northern Ireland, and legal experts.
"The vast majority of these groups believe that implementing a statute of limitations on criminal cases, Ombudsman reports, inquests, and civil actions, alongside legacy mechanisms without investigations by a Historical Investigations Unit, will prevent all genuine paths to justice – justice which is still being achieved.
"In fact, there are a number of outstanding cases remaining, many of which would be directly impacted by this decision. Those victims and their families, from all communities, still desire justice and more importantly, they deserve justice."
The Congress members cited the recent conviction of a former British soldier for the manslaughter of 23-year-old Aidan McAnespie, who was shot at a border checkpoint at Aughnacloy, Co Tyrone, in 1988.
"This case is a testament to the continued and necessary judicial process that this Bill impedes," they wrote.
"As such, we believe that for as long as outstanding evidence and potential criminal liability remains, avenues to justice must remain open.
"Prime Minister Sunak, as we approach the 25th anniversary of the Belfast/Good Friday Agreement we call on you and your Government to assess the impact of this legislation which directly and unilaterally impacts the peace and stability across the island of Ireland.
"We strongly urge you to return to values agreed upon in the Belfast/Good Friday Agreement and ensure victims of violence from the Troubles receive justice and perpetrators of crimes committed during the Troubles are held accountable."
Ms O'Neill welcomed the intervention from the US politicians.
"There is deep frustration that the British Government's Legacy Bill which will halt in law the rights of all victims to access truth, justice and accountability is not being heard by the British Government," she said.
"The criticism of the Bill, both domestically and internationally, has been widespread and consistent.
"This bipartisan letter from members of Congress directed to Rishi Sunak is a significant intervention which must not be ignored."
During the last week, the UN High Commissioner for Human Rights said the Bill must adequately ensure respect for the rights of victims, survivors and their families.
Volker Turk said the draft legislation as it stood appeared to be incompatible with the UK's international human rights obligations.
A spokeswoman for the UK Government said: "The UK Government is determined to deliver better outcomes for all those most impacted by the Troubles while helping society to look forward.
"The Northern Ireland Troubles (Legacy and Reconciliation) Bill seeks to deliver a significantly different and improved approach to addressing the legacy of the Troubles in Northern Ireland, focused on providing more answers to as many families as possible.
"Truth and reconciliation is at the heart of these proposals – the new body will provide answers to all who want them, supported by full disclosure by the state.
"We have tabled a number of proposed amendments to the Bill which seek to address key concerns that have been raised by many stakeholders, including victims and survivors.
"These amendments reflect the extensive engagement that has taken place, and demonstrates the Government's commitment to working with all interested parties regarding their concerns, and how these might be addressed."