David Cameron is promising Scotland more powers to govern itself if it votes against independence in a referendum - though he is refusing to say exactly what he would be prepared to hand over.
Britain's prime minister made the offer before meeting Scotland's first minister Alex Salmond.
It was the first time the two men have met since a row broke out between Westminster and Holyrood over the proposed referendum on Scottish independence.
"If the prime minister has an offer to make to the people of Scotland, then he should make it now," Salmond told the BBC before his meeting with Cameron.
"He should spell it out now so we can have a clear decision on the alternative futures for Scotland.
"This idea of saying, well, vote no and we'll give you something later, I don't think is going to convince anyone in Scotland."
Senior Scottish National Party figures have accused the British government of interfering in Scottish politics, by saying it may impose a time-limit on when the referendum must take place, as well as how the question would be phrased on the ballot paper.
Scotland must also be lent referendum powers by Westminster.
"The Westminster government should accept that this is a decision not for them, but for the Scottish government and the Scottish people," deputy first minister Nicola Sturgeon said.
A decision needs to be made over how to pitch the independence question to the Scottish electorate.
There is a third option being proposed, which would see Scotland remain a part of the union, but have independence from Westminster over its fiscal and social policies.
This option, known as "devolution-max", is favoured among Scottish voters, according to opinion polls.
Critics say this would reduce the mandate of any referendum outcome, as it splits the vote further than a simple two-option ballot.
The House of Lords constitution committee said all UK voters, not just the Scottish, should be allowed to vote on devolution max.
It also called for an in-out only referendum.
"We are firmly of the view that any referendum held must be a straight choice between full independence or the status quo," committee chairwoman Lady Jay said.
"A third devolution-max option is clearly something every part of the UK must have a say in, as it has the potential to create different and competing tax regimes within the UK," she added.