Theresa May has written to a committee of senior Irish-American leaders, who have expressed concerns about Brexit and protecting the Good Friday Agreement (GFA).
The former members of congress contacted the prime minister and Taoiseach (Irish prime minister) Leo Varadkar last month.
They said it was clear some British politicians had "little knowledge" of the importance of the agreement.
The peace deal was signed in 1998.
The agreement, also referred to as the Belfast Agreement, helped end decades of violence in Northern Ireland.
It also enshrined the birthright of all people in Northern Ireland to be recognised as Irish, British or both - but some human rights groups have said a no-deal Brexit could threaten those rights.
Former Republican congressman James T Walsh, who co-chairs the Protecting the Good Friday Agreement ad hoc committee in the US, has now published the letter he received from Mrs May.
In it, the prime minister said the government's commitment to the 1998 agreement remains "steadfast - to the principles it embodies, the political institutions it seeks to establish and the rights that it guarantees".
"No government that I lead will ever take risks with the hard won relative peace and stability that these agreements have established," she added.
The US group had expressed concerns about recent votes in the House of Commons, where the government's Brexit deal was rejected because of the Irish border backstop: the insurance policy to maintain an open border unless and until another solution is found.
But the prime minister said: "While I appreciate the concerns expressed over the recent vote, the commitments we have set out remain unaffected."
Mrs May also said the Northern Ireland Secretary, Karen Bradley, would welcome a discussion with a delegation from the United States when she is in Washington later this month.
Bruce Morrison, the other co-chair of the congressional committee, said he would be happy to meet Mrs Bradley.
"Brexit has taken the oxygen out the politics of Northern Ireland and any fall off in support for the Good Friday Agreement would only make matters worse," he added.
The committee also said if there is a no-deal Brexit, "Irish America would work closely with its allies" in congress to shore up the Good Friday Agreement in any future trade negotiations between the United States and Great Britain."