The DUP leader has said it is “simply not the case” that her party is boycotting north-south political meetings in protest at the Northern Ireland Protocol.
Arlene Foster’s comments came as other executive parties held a series of meetings on Monday to discuss two DUP no-shows at sectoral meetings of the North South Ministerial Council in recent weeks.
As Mrs Foster was addressing the Assembly, Sinn Fein President Mary Lou McDonald told a Stormont press conference that the DUP could not “cherry pick” from the Good Friday Agreement.
Mrs McDonald urged the party to clarify whether it will participate in future north-south political meetings.
SDLP leader Colum Eastwood also accused the DUP of “childish” behaviour.
The latest incident unfolded on Friday, when SDLP minister Nichola Mallon said a meeting with Irish transport minister Eamon Ryan was cancelled at the last minute after the DUP failed to nominate a unionist minister to attend.
One accompanying minister from the opposing political view must attend North South Ministerial Council meetings to allow them to proceed.
In February, the DUP warned that north-south relationships will be “impacted” as part of its political campaign to oppose the Northern Ireland Protocol, which governs post-Brexit trading arrangements on the Irish Sea.
The party also pledged to oppose all protocol-related measures in the Northern Ireland Assembly.
The two recent occasions when the DUP did not participate in north-south meetings were for engagements unrelated to Brexit and the Protocol.
Mrs Foster denied her party was “refusing” to attend meetings between the Northern Ireland Executive and the government of the Republic of Ireland.
“I spoke with the Taoiseach on Friday week ago by telephone,” she told MLAs.
She added: “Just to be clear, the Northern Ireland Protocol is not part of the North South Ministerial Council.
“As we have outlined some weeks ago, north-south relationships will of course be affected by the fact that the Protocol is in place because it has damaged east-west relationships and we need to sort it out.
“We need to get rid of the protocol so that we can continue to have relationships with our nearest neighbour in the appropriate way in the future so that the constitutional and economic position of Northern Ireland is something that every community can benefit from as opposed to the dreadful situation that we currently find ourselves in.”
Mrs Foster added: “I have made it clear at all times that I and colleagues will act in the interests of the people of Northern Ireland and in accordance with all of our ministerial responsibilities and indeed the pledge of office as we have indicated.”
Mrs McDonald was in Belfast for meetings with other Stormont Executive parties.
A meeting with the DUP did not happen but the Sinn Fein leader insisted it would take place in the near future.
You can't cherry pick the Good Friday AgreementMary Lou McDonald, Sinn Fein
“You can’t cherry pick the Good Friday Agreement,” she said.
“Political progress, stability and inclusion relies on all of the institutions of the Good Friday Agreement working and working well.
“The north-south piece is absolutely essential and that means that unionists of all hues have to participate in those meetings.
“I think it would obviously be hugely problematic if there was any suggestion that any strand of unionism was stepping back from that or causing a problem with the north-south mechanism.”
In terms of future north-south ministerial meetings, she added: “We fully expect that the DUP, that political unionism, will play its part and meet its obligations in respect of the Good Friday Agreement.
“You cannot opt out of parts of the Good Friday Agreement. We absolutely expect that the north-south meetings will happen.
“We expect that unionism will participate in those meetings. And I have to say anything short of that raises very serious questions, not just for the DUP, but for all of us. So I hope that they’ll clarify that position.”
Mr Eastwood said it was “disappointing” that the DUP had not met his party on Monday to discuss the issue.
There's a legal obligation to take part in north-south ministerial meetingsAlliance MLA Andrew Muir
“I think this in-and-out approach in terms of their engagement with political institutions is childish and it needs to stop,” he said.
He added: “There is no Good Friday Agreement if one of the strands (north-south engagement) is taken away.
“So it’s very important, it’s absolutely fundamental to the operation of these institutions.
“And we are absolutely insistent that all aspects of the Good Friday Agreement are properly and fully working.”
Mr Eastwood continued: “This isn’t a moment for escalating things into a crisis situation, what we want is people to come together, to sit down and to work through the issues.”
Alliance MLA Andrew Muir added: “Decisions made last week to block north-south ministerial meetings taking place is regrettable.
“There’s a legal obligation to take part in north-south ministerial meetings.
“Really the solution to these things is more co-operation rather than less, and the politics of veto and blocking things is not what Northern Ireland needs at this time.”