The Tories aim to cut £12bn from the welfare budget by 2017-18.
Documents leaked to the BBC suggested taxing disability benefits was among the options - but the work and pensions secretary said none were party policy.
Meanwhile Labour's campaign chief has indicated it will not outline further plans for cuts before the election.
Asked if the party would be making any more substantial promises on cuts during the campaign, Douglas Alexander told the BBC's Andrew Marr Show: "We've set out our agenda and that's the programme on which we will fight the election."
Last week's leaked documents from Mr Duncan Smith's department outlined some options for cutting benefits further, prepared by civil servants at the request of Conservative Party officials.
They included restricting Carer's Allowance, taxing disability benefits, limiting child benefit to the first two children and regional variations in benefit caps.
Mr Duncan Smith told the Andrew Marr Show "none of the stuff" that had been reported had been discussed with the chancellor, and it was standard for government departments to put figures together - but that did not mean they were policy.
He said a benefit freeze and pledge to reduce the annual benefits cap from £26,000 to £23,000 would account for about a quarter of savings and were "a good indication that we know where we're going to go to make those savings".
He said he had raised Carers' Allowance and had protected the most disabled throughout all previous benefit changes.
Mr Duncan Smith said it was "not the case" that planned cuts were being kept secret - but Labour urged him to "come clean".
"I didn't come into this job after years looking at this to just make cheese-paring cuts," he said.
"What we've come in to do is to reform the welfare system, so that we don't waste money on organisations and groups and things that don't actually help life change.
"No decisions have been made. As and when decisions are made of course we will be very open to the public."
He added: "I know that it is feasible to save £12bn, it is less than 10% of the overall budget... There will be loads and loads of things that people are looking at, because that's what departments do."
Mr Duncan Smith said changes made so far had improved people's lives - contrary to predictions that they would result in much homelessness and people having to move hundreds of miles away.
The government has cut around £20bn from projected welfare spending over the course of the past five years, through a range of measures from freezing payment rates to cutting housing benefit.
The Conservatives have said they would freeze the rate at which benefits are paid to people of working age and lower the benefits cap - the amount a household can claim in a year - to £23,000, if they win the general election.
The independent Institute for Fiscal Studies suggests those measures would save no more than about £2bn a year by 2017-18.
Shadow work and pensions secretary Rachel Reeves said Labour had a better plan to "control the costs of social security" by tackling low pay and housing costs, and said the Conservatives must "come clean" on where they would cut.
She added: "Iain Duncan Smith's refusal to admit how children, disabled people, carers and working families will be hit by secret Tory plans six weeks before the election is completely unacceptable."
But the Conservatives said Labour's economic policies were "not credible" as Douglas Alexander had failed to spell out further details of where Labour would cut spending or increase taxes.

The 64-year-old minister, who has a history of chronic diabetes, was admitted to a hospital in Delhi earlier this month.
She tweeted to her followers on Wednesday that she was currently on dialysis.
Ms Swaraj is one the most high-profile ministers in Prime Minister Narendra Modi's cabinet.
Officials at the hospital told the PTI news agency that "chronic diabetes has affected her kidney functioning but her condition was stable".
She is known for reacting quickly to calls for help from Indians living or travelling abroad.
Earlier this month, she helped a Pakistani bride get a visa to attend her wedding in India.
No decisions have been made on where to make further benefit cuts, should the Conservatives win the general election - Iain Duncan Smith has told the BBC.
[NEXT_CONCEPT]
Indian Foreign Minister Sushma Swaraj has said that she is undergoing tests for a kidney transplant.