Paula Dobriansky, the top State Department official for global affairs, will become special US envoy for Northern Ireland, replacing Ambassador Mitchell Reiss.
The transition will take place on February 15th. Ms Dobriansky will retain her existing duties.
A State Department announcement said that President George Bush and Secretary of State Condoleezza Rice were fully committed to the implementation of the 2006 St Andrews Agreement.
The agreement requires Sinn Féin to accept policing and justice arrangements while the DUP must agree to share power with the republican party in the Stormont Executive.
"Under Secretary Dobriansky will continue the US commitment to peace and reconciliation in Northern Ireland at a time of renewed confidence in the process," State Department spokesman Sean McCormack said.
Meanwhile, Sinn Féin leaders have suffered a setback in their bid to persuade party members to support the PSNI, it emerged tonight.
The party's youth wing Ogra Shinn Féin has instructed delegates to this Sunday's special conference in Dublin to vote against party leader Gerry Adams' motion which recommends Sinn Féin get involved in policing structures in Northern Ireland.
However the party leadership was quick to point out that while the youth wing decided to vote against the motion, they also agreed to respect whatever decision was made at this weekend's meeting.
Barry McColgan, the head of Ogra Shinn Féin, said: "The issue of policing is absolutely huge for republican activists, our communities and support base and any decision taken on policing will have wide ranging implications for republicanism as a whole.
"Ogra Shinn Féin's position from its inception since 1997 has always opposed the brutal, sectarian state militia, that passed for a police force in the north.
"It was Ogra who spearheaded the 'Disband the RUC' campaign, it was Ogra who called on young people not to join in a repackaged RUC - the PSNI; and it was Ogra who last year were shouting from the rooftops, literally, to 'smash political policing'.
He added that despite the opposition party unity was more important.
Sinn Féin leadership figures have been involved in a frantic round of internal debates and public meetings over the past 10 days which will continue in the run-up to this Sunday's policing debate.
Local branches are currently deciding how they will approach the debate. However despite Ogra Shinn Féin's rejection of the motion, leadership sources were satisfied with how the debate within the party was unfolding.
"The important thing about the Ogra position is they have said they will respect the verdict of Sunday's extraordinary ard fheis," a source said.
"What is quite clear from the public meetings that Gerry Adams and others have been conducting is that people are coming to this issue in a mature fashion and they are coming out agreeing that no matter what view they take republicans will stay united," the source added.
Sinn Féin support for policing in Northern Ireland is seen by Taoiseach Bertie Ahern and Prime Minister Tony Blair as crucial to their plan to set up a power sharing government in Northern Ireland by March 26th.
AP