Secretary of State for Northern Ireland James Brokenshire has called for a respectful election in Northern Ireland.

Former first minister Arlene Foster has predicted the contest would be "brutal".

But Mr Brokenshire told MPs in the House of Commons the future of power-sharing was at stake.

He was forced to announce a poll yesterday after Sinn Féin's Martin McGuinness resigned as deputy first minister in a row with coalition partners the Democratic Unionists over an overspending eco-boiler scheme.

Mr Brokenshire said: "This election is about the future of Northern Ireland and its political institutions.

"Not just the Assembly but all of the arrangements that have been put in place to reflect relationships throughout these islands.

"That is why it will be vital for the campaign to be conducted respectfully and in ways that do not simply exacerbate tensions and division.

"Once the campaign is over we need to be in a position to re-establish strong and stable devolved government in Northern Ireland."

The US has urged a quick resumption to power-sharing in Northern Ireland.

In a statement, the State Department said: "The United States remains committed to supporting a more peaceful and prosperous Northern Ireland.

"To this end, we urge all political parties to focus on quickly finding a way forward to the resumption of stable devolved governance.

"We also encourage civil society leaders to continue their vital work building a better, shared future.

"Restored devolved democratic institutions and an engaged, constructive citizenry can best take Northern Ireland forward."

Meanwhile, Ulster Unionists have referred the Renewable Heat Incentive, which threatens to cost the taxpayer up to £490 million over the next 20 years, to the police.

Some boilers are producing profits of more than 80% for users, the Stormont assembly was told.

MLA Steve Aiken said: "I have written to the Chief Constable of the PSNI (George Hamilton) on foot of assertions made by the Minister for the Economy at yesterday's emergency economy committee meeting."

A PSNI statement confirmed it had received Mr Aiken's letter and was considering the contents.