Under growing pressure from rival Republicans to release his tax returns, Mitt Romney said on Tuesday that he paid a tax rate approaching 15 percent on his millions of dollars in annual income but said he would not make public his full return until April.

His effective tax rate was “probably closer to the 15 percent rate than anything,” Mr. Romney said at a campaign stop in South Carolina, noting that most of his considerable income over the last decade has come from investments rather than from earned income like salary. He also characterized as “not very much” the $374,327 he reported earning in speaking fees last year, though that sum would, by itself, very nearly catapult most American families into the top 1 percent of the country’s earners.

Mr. Romney mentioned the 15 percent figure in response to a question at a news conference, leaving uncertain whether his comment was unplanned or a deliberate effort to begin airing the issue ahead of a general election campaign. But his remarks on Tuesday drew immediate criticism from the Obama administration and Republican rivals, and are likely to cement Mr. Romney’s place as an unwilling emblem of the intensifying national debate over taxation and income inequality, which burst into the campaign this month when rival candidates began attacking Mr. Romney’s career in the leveraged buyout business.

Mr. Romney’s admission left unclear — and his campaign declined to clarify — whether the 15 percent he was referring to represented his overall tax burden or simply his federal income taxes. That makes it hard to compare the figure with the tax burdens of a typical middle-class American — or of Mr. Romney’s opponents. President Obama reported paying an effective federal tax rate of 26 percent on his 2010 family income.