Hillary Clinton has urged an end to the new outbreak of violence in Northern Ireland, saying: "It is never an acceptable response to disagreements."

Her call came after a death threat was issued against MP Naomi Long, who was warned by police to stay away from her home and constituency office, where loyalists staged demonstrations in protest against a decision by the city council to limit the flying of the Union flag.

"All parties need to confront the remaining challenge of sectarian divisions, peacefully together," said the US secretary of state, who was meeting First Minister Peter Robinson and Deputy First Minister Martin McGuinness at Stormont Castle.

Talks on the economy and the peace process were overshadowed by loyalist tensions on the streets and the discovery of another dissident republican bomb in Londonderry, where police arrested four men.

This is her second visit as secretary of state. The last was in 2009 when she encouraged the devolution of policing and justice powers from London to Belfast.