But she told MPs that ministers' plans to build three million new homes by 2020 were an "ambition" rather than a strict target.
Tories jeered her comments at Commons question time, as they pressed Mrs Beckett on the effects of the economic downturn on housing policy.
The Prime Minister announced plans to build three million new homes by 2020 last July when he put affordable housing at the top of the Government's agenda.
Shadow housing minister Grant Shapps said: "Now you have dumped the Prime Minister's three million by 2020 target for house building and said it was only ever an ambition - when it certainly was a target according to the quotes I have from the Prime Minister here - can you tell us whether eco-towns were also only ever an ambition?
"Or does the faltering eco-town project remain on target for 10 new eco-towns?"
Mrs Beckett said: "The eco-towns programme is totally on course.
"We've only recently published the the draft planning guidance and the sustainability appraisal.
"I am not committed to any specific number. In the programme that was put forward before, one site has met the generally suitable criterion, others we believe can meet it. But they all have ambitious and difficult targets to achieve and they will be different for each one."
She said the standards for eco-town status would not be lowered. "That's the reason I am saying, I am not and nor is the Government, nor have we ever been, fixed to any specific number.
"It will depend on what meets the targets. But I will be extremely disappointed if there aren't substantially more than you are trying to pretend."