The Ulster Unionists have called on Michelle O'Neill to state that Sinn Fein unambiguously supports the Good Friday Agreement following comments from her local MP branding it "a bluff".
Francie Molloy tweeted: "We were sold a pup with the GFA - no commitment from either Dublin or London to deliver for nationalists or republicans, it was just a bluff."
The SDLP described his remarks as "deeply worrying" and Alliance also expressed concern.
A Sinn Fein spokesperson last night said: "Sinn Fein fully supports the Good Friday Agreement - in all of its parts - and is committed to its full implementation including the provision for a referendum on Irish unity."
Speaking earlier, UUP leader Steve Aiken said: "Francie Molloy's comments have raised serious questions about Sinn Fein's commitment to the Belfast Agreement. Sinn Fein has spent years making public statements about the need to 'defend the Belfast Agreement' yet at the weekend, the Mid Ulster MP said that nationalists had been 'sold a pup' and described the Belfast Agreement as 'just a bluff'.
"Either Sinn Fein supports the Agreement or it doesn't and Michelle O'Neill needs to clarify this as a matter of urgency."
Mr Aiken said that Mr Molloy's frustration was heightened by the fact that Sinn Fein's demand for a border poll had been "rejected out of hand by all three governing parties in Dublin - Fianna Fail, Fine Gael and the Greens".
He added: "Given that a few weeks ago Sinn Fein was boasting of how it expected to provide the next Taoiseach, it may be that some in the party are having difficulty adjusting their expectations to meet uncomfortable new political realities."
Alliance deputy leader Stephen Farry said: "Sinn Fein need to clarify their position on this comment from Francie Molloy.
"The Good Friday Agreement sets out the fundamental terms for any border poll. It doesn't say there shall be one or specify when it should take place. This was as clear in 1998 as it is today. All parties should work to build on the Good Friday Agreement, including developing relationships across the island of Ireland."
SDLP leader Colum Eastwood described Mr Molloy's comments as "deeply worrying" and said he hoped they weren't representative mainstream Sinn Fein. "If people in that party are dissatisfied with the progress that has been made, they would be better to reflect on their 13 years as joint head of government rather than blaming the institutions of peace," Mr Eastwood said.
"The only path to uniting the people of this island is through the spirit of partnership, cooperation and reconciliation that the Good Friday Agreement is built on.
"Delivering a new Ireland demands that we all spill our sweat to forge new enduring relationships between the people of this island and that we demonstrate to everyone that there is an equal place for them. That's the challenge that the agreement sets for those of us who support Irish unity. It's no bluff."
The SDLP leader said his party was "prepared to spill our sweat again" in building relationships and setting out its vision of Irish unity. "Rather than thrashing around in blame, others should join us," he added.
Mr Molloy told the Belfast Telegraph that his tweet had been written "out of frustration" after Taoiseach Micheal Martin dismissed the idea of a border poll.
The Mid Ulster MP said he believed any reticence about holding a referendum was due to fears that a majority of people would vote in favour of a united Ireland.
"I think that after Brexit, the governments do actually realise that it could happen," he said.
"My comments are down to the fact that the Irish Government are signatories to the Good Friday Agreement, yet they aren't doing anything to implement the border poll. Both Dublin and Westminster signed up to the Good Friday Agreement, they're both guarantors, yet Micheal Martin is dismissing the idea of asking for a referendum for Irish unity," he said.