US President Barack Obama has released his tax return for 2011, as he seeks to make taxation a key election issue with his Republican foes.

The return says Mr Obama paid an effective tax rate of 20.5% on total income of nearly $790,000 (£498,000).

The president has recently advocated making wealthy Americans pay more tax.

Analysts say Mr Obama is attempting to characterise Mitt Romney, his wealthy Republican rival, as remote from ordinary economic concerns.

On the same day that Mr Obama released his tax information the Romney campaign said that their candidate had filed for an extension on his 2011 tax return.

Mr Romney, a wealthy businessman, is likely to pay 15.4% in tax on an income of $20.9m in 2011, according to an estimate he has released.

Correspondents say Mr Romney pays a lower rate because the US tax system treats income from investments more favourably than wage earnings.

Half of Mr Obama's $789,674 income in 2011 came from his presidential salary, with the remainder from sales of his books, according to the return published by the White House.

Mr Obama and fellow Democrats have renewed calls for the rich to pay a "fair share", ahead of a vote in the US Senate on the so-called "Buffett Rule".

Under the proposal before the Senate, named after billionaire investor Warren Buffett, there would be a minimum 30% tax on earnings over $1m.

Mr Buffet has often pointed out that he pays a lower effective tax rate than his secretary.

The White House confirmed that Mr Obama also paid a lower tax rate than his secretary, adding that the anomaly demonstrated why the US tax code was in need of reform.