The US State Department is watching developments in Northern Ireland "very carefully" and has urged all political parties to "engage constructively" in talks.

State Department spokesman John Kirby said former US senator Gary Hart, Secretary of State John Kerry's personal representative on Northern Ireland, remained "closely engaged" with the parties and the Irish and British governments.

“Mr Hart is in constant contact with all the authorities,” said Mr Kirby. He declined to say whether Mr Kerry would send Mr Hart to Northern Ireland in response to the crisis. “I am not going to speculate as to future activities,” he said. “We want all the parties and leaders to work together constructively.”

According to Taoiseach Enda Kenny, the SDLP has suggested the US might take greater involvement, with Mr Hart being appointed to a more enhanced role by the Obama administration.

Mr Hart said last week that he had “strongly encouraged” all the parties in recent conversations to engage in intensive talks.