Image copyright Sinn Féin Image caption Martin McGuinness has been meeting US officials over the last three days in the hope they will lobby the UK government to provide more financial support to Stormont

Martin McGuinness has denied that his three-day visit to the United States for talks about the Stormont budget crisis has been a waste of time.

He said if the crisis was not resolved by September, Stormont's government would be "hanging by a thread".

The Sinn Féin MLA has faced criticism from unionists for meeting US officials instead of holding more talks at home.

But said he would not have been invited to US meetings if White House officials believed the talks were pointless.

During his trip, Northern Ireland's deputy first minister has met representatives from the US State Department and influential Irish Americans, asking them to lobby the British government to give more financial support to Stormont.

His trip has coincided with a diplomatic visit to the US by Northern Ireland Secretary of State Theresa Villiers, who has said a budget increase is not an option.

On Tuesday, Democratic Unionist Party (DUP) Minister, Arlene Foster, described Mr McGuinness' US mission as a "waste of time".

However, he disagreed and said he had "come away very encouraged" by what he had heard from Barack Obama's administration.

"The fact is Theresa Villiers is here as well. Does Arlene consider that Theresa Villiers' trip is a waste of time?" Mr McGuinness said.

"Certainly the White House [staff] don't think it's a waste of time. I wouldn't have been invited to meet with a key official if that had been the case."

The Sinn Féin MLA said he was "working flat out" to achieve a resolution to the welfare reform crisis that has engulfed Northern Ireland's political institutions for months.

He said he had made the US government aware of the seriousness of the crisis threatening Stormont's future and the likely timescale.

"The institutions, if we don't resolve this problem by September, are effectively hanging by a thread," he told BBC Radio Ulster.

Mr McGuinness said the money required to resolve the welfare budget row was a "drop in the ocean" compared to the resources that British governments had spent "pacifying" Northern Ireland during the Troubles.