The Caledonia Together party says it won't vote for the presidential candidate of the rival anti-independence Future with Confidence group, thereby threatening a possible government deadlock.
Future with Confidence is expected to get five ministries in the 11-member collegial government in Thursday's Congress meeting which is the same number as the pro-independence side.
After losing more than half of its Congress seats in last month's election, Caledonia Together is entitled to one ministry but senior leaders say the party won't vote for the likely Future with Confidence presidential candidate Thierry Santa.
If no candidate can secure at least six of the 11 ministerial votes, there can be no new government and the old government will stay on in caretaker mode.
The apparent stalemate could end if Caledonia Together changes its stance, the pro-independence side votes for Mr Santa or Future with Confidence votes in a pro-independence presidential candidate.
After the last election in 2014, it took months to overcome a similar stalemate caused by dissent within the anti-independence camp.
The newly formed Pacific Awakening party which has joined Future with Confidence for the formation of government has been campaigning for end to the long-running political rift over independence from France.