UPDATE 1 — UK PM Burnham and Irish PM Martin reaffirm commitment to Good Friday Agreement

(Adds quote, background in paragraphs 2, 4-5) LONDON, Sept 18 (Reuters) — Britain's Prime Minister Andy Burnham and Irish leader Micheal Martin on Friday reaffirmed their commitment to upholding the Good Friday Agreement, Downing Street said in a statement issued after the two leaders met in Manchester. The 1998 Good Friday Agreement ended three decades of violence between Irish nationalists seeking a united Ireland and unionists who wanted Northern Ireland to remain part of the UK. "The two leaders began by reaffirming the close partnership between the UK and Ireland, as well as their shared commitment to upholding the Good Friday Agreement," a Downing Street spokeswoman said. The comments come after US President Donald Trump raised the question of Irish unification up the political agenda during a visit to Ireland last week, saying he would love to see Ireland and Northern Ireland unified. Under the terms of the GFA, a referendum on unification can be held only at the discretion of the British government. Trump's intervention followed Burnham saying a referendum on Northern Ireland leaving the UK was "off the table", which drew an angry response from leaders of Irish nationalist