Alex Salmond said progressive politicians could "seize the moment"
The Scottish National Party has called on the Liberal Democrats to join a "progressive alliance" involving Labour, the SNP and Plaid Cymru.
SNP leader Alex Salmond urged the Lib Dems to consider the alliance as an alternative to a deal with the Tories.
Senior Liberal Democrats have been meeting to discuss the power-sharing offer issued by the Conservatives.
An arrangement between Labour and the Lib Dems, SNP and Plaid Cymru could command a majority in the Commons.
Mr Salmond told the BBC: "The assumption is a Tory/Liberal Democrat pact is not correct.
An arrangement between Labour, the Lib Dems, the SNP and Plaid could command a majority in the House of Commons. The nationalist parties would, of course, extract financial and political concessions from Westminster
"There are alternative and more progressive options available if politicians have the will to seize the moment. The SNP and Plaid are indicating that we do."
The arrangement would still rely on the traditional voting patterns of the Northern Irish parties.
Labour dismissed the SNP's progressive alliance suggestion as a desperate attempt by Mr Salmond to make himself look relevant.
The Tories won most votes in the UK election but were short of a majority.
The Conservative and Lib Dem negotiation teams will meet again on Sunday and there will be a meeting of Conservative MPs on Monday, the BBC understands.