Emma Little-Pengelly has accused Alliance leader Naomi Long of being "petty" and trying to score political points over the DUP MP's support of a review into how the UK helps persecuted Christians around the world.
South Belfast MP Ms Little-Pengelly welcomed news that Foreign Secretary Jeremy Hunt has ordered a review into the plight of Christians around the world.
"Persecution of Christians across the globe is on the rise," she tweeted. "Jim Shannon MP has been at the forefront of this campaign as Chair of the APPG [All-Party Parliamentary Group] at Westminster on Religious Tolerance. Welcome news."
In response, East Belfast MLA Mrs Long noted two countries on the Open Doors World Watch List top 50 where Christians face "the most extreme persecution" - the Maldives at number 13 and Sri Lanka at number 44.
DUP MP Ian Paisley has faced criticism for taking holidays in both countries, and lobbying on their behalf.
Open Doors is an international ministry which serves persecuted Christians around the world, supplying Bibles and support.
Mr Paisley was recently suspended from the Commons for 30 sitting days for serious misconduct. The Parliamentary Commissioner for Standards had found that he had failed to properly declare two family holidays to Sri Lanka in 2013 and engaged in paid advocacy for the Sri Lankan government.
More recently, he has denied any wrongdoing after it was reported that he was given a complimentary holiday at a luxury Maldives resort months after advocating on behalf of its government. Mr Paisley claimed the trip in 2016 was a private family vacation paid for by himself and a close friend unconnected to his work. He has said he is satisfied that he did not have to declare the holiday.
Referring to Mrs Long's tweet, Ms Little-Pengelly said she had been "100% consistent" and had supported Open Doors. She added: "I feel deeply passionate in this cause. You are making a petty political dig against the wrong person here. We should all be welcoming this news, not trying to score political points..."
Mrs Long has not yet responded to Ms Little-Pengelly. However, asked if she supported the review, the former East Belfast MP said she was a parliamentary representative for Open Doors and was vice chair of the APPG for Freedom of Religion and Belief for four years.
"I take freedom of and freedom from religion and belief extremely seriously, whether abroad or at home," she said.
The Foreign Office review comes as reports of violence against Christians are rising, with an average of 250 killed a month. Mr Hunt said the UK "must do more" and the review will make ambitious policy recommendations.
It comes after outcry over the treatment of Asia Bini, a Christian woman who faced death threats after being acquitted of blasphemy in Pakistan.
Welcoming the review, DUP MP Jim Shannon said: "In a country where we have religious liberty to practice our faith, it seems unimaginable that in 2018 people are put to death just because they worship the Lord Jesus Christ."