DUP leader Peter Robinson has said his party will not support a Westminster government that is "captive to a separatist party".
Mr Robinson was asked by the BBC if his party would join forces with the SNP following the General Election on 7 May.
He said: "We are unionist, we want to retain the UK.
"If a government was being propped up by a separatist party that was using its position in order to extract levers of separation, then clearly we could not support that," he said.
"If the ask of any party that forms such a support group for a government party is for purely regional issues then that is a very different matter indeed".
Last weekend, the DUP issued a five-point plan it will use as part of any negotiations in the event of a hung parliament after the election.
It includes a demand to scrap the so-called bedroom tax.