Theresa Villiers has refused to be drawn on whether an EU exit would be good for Northern Ireland amid concerns over a split within her team at the Northern Ireland Office.
The Northern Ireland Secretary is reported to be considering campaigning against the UK's continued membership of the European Union during the forthcoming referendum.
In contrast, Northern Ireland minister Ben Wallace told MPs he believed a reformed EU was "where the UK wants to be".
Labour urged Mr Wallace to reassure the Stormont Executive and Northern Irish public about the region's future within the EU given the "mixed messages" from him and his ministerial boss.
Meanwhile, DUP MP Sammy Wilson tried to push Ms Villiers to back a British exit - referred to as Brexit - as he noted the positives of leaving.
The MP for East Antrim asked Ms Villiers: "While the devolution of corporation tax will be important in growing the Northern Ireland economy, would you also agree a vote to leave the EU would help the Northern Ireland economy insofar as it would release £18bn every year for expenditure on public services, would enable us to enter trade agreements with growing parts of the world, and would release us from the stifling bureaucracy of Europe?"
Ms Villiers replied: "Well, I'm afraid you're tempting me to engage in arguments which are rightly a matter for everyone in this country when they get to vote on that referendum.
"We promised a referendum in our manifesto - that's what we're going to deliver."
Margaret Ritchie MP earlier asked Mr Wallace: "Would you encourage your colleague (Ms Villiers) to argue for a yes vote in the EU referendum?"
Mr Wallace replied: "I think there's a temptation in front of me. I think what I'd say is, certainly up to date, membership of the European Union has been good for Northern Ireland. I hope and support the Prime Minister's efforts in achieving reform.
"A reformed EU is where the United Kingdom wants to be - an EU that works to the benefit of everyone in the United Kingdom - and I think if we can achieve that then we can take advantage of being neighbours of Ireland - one of the biggest economic partners of Northern Ireland - to make sure the economy can go from strength to strength."
Shadow Northern Ireland Secretary Vernon Coaker also warned there was "very real concern" in the region about the impact of leaving the EU on trade and investment.
He told the Northern Ireland minister: "You will know there's very serious concern in Northern Ireland about a possible Brexit, in particular because of the fact that it's the only part of the UK with a land border with another EU country.
"Can I also (press you) to reassure the Executive and the people of Northern Ireland about this in view of the mixed messages on Brexit emanating from the ministerial team - in particular your views as opposed to the secretary of state."
Mr Wallace replied: "There is no mixed message. Both myself and (Ms Villiers) are keen for the EU to produce some reforms, as is the Prime Minister's strategy. And when those reforms - and maybe you know what the EU is agreeing, maybe you have a special hotline - are presented to this House, then we'll be able to make a decision.
"For my part, I believe in the past that membership of the EU has been good for Northern Ireland."