Uganda's President Yoweri Museveni has refused to approve a controversial bill to toughen punishments for homosexuals.

He has written to the parliamentary speaker criticising her for passing it in December without a quorum.

Homosexuals were "abnormal" or were so for "mercenary reasons" and could be "rescued", a local paper quotes his letter as saying.

The bill includes life imprisonment for homosexual acts and also makes it a crime not to report gay people.

The promotion of homosexuality - even talking about it without condemning the lifestyle - would also be punishable by a prison term.

The BBC's Catherine Byaruhanga in the capital, Kampala, says the president is aware that if he signs the bill there will be an international outcry, which could see some countries suspend aid to the country.

Mr Museveni is trying to reach a compromise with MPs, as if he refuses to sign the bill, parliament can still force it through with a two-thirds vote.

But in contrast to Nigeria, where earlier this month the president signed a bill banning same-sex marriages, gay groups and shows of same-sex public affection, Mr Museveni is politically strong and so more able to resist pressure from conservative groups, our reporter says.

'Sexual starvation'

Mr Museveni said the bill was forced through despite his advice to shelve it until the government had studied it in depth, Uganda's private Monitor newspaper reports.

"Even with legislation, they will simply go underground and continue practicing [sic] homosexuality or lesbianism for mercenary reasons," he is quoted as saying.

The president's eight-page letter to speaker Rebecca Kadaga said they could be "rescued" by improving the economy.

He also disputed the view that homosexuality could be described as an "alternative sexual orientation".

"You cannot call an abnormality an alternative orientation. It could be that the Western societies, on account of random breeding, have generated many abnormal people," he said.

He said another reason women became lesbians was because of "sexual starvation" when they failed to marry, the Monitor reports.

There is meant to be a caucus meeting of ruling party MPs later this month to discuss the bill.

The government will try to convince them to reject it, but some have already said they would go against their government's wishes, our correspondent says.

Human rights activists say the bill highlights the intolerance and discrimination the gay community faces in Uganda.

One gay activist was killed in 2011, although the police denied he was targeted because of his sexuality.

The bill has been condemned by world leaders since it was mooted in 2009 - US President Barack Obama called it "odious".

The private member's bill originally proposed the death penalty for some offences, such as if a minor was involved or the perpetrator was HIV-positive, but this clause has been dropped.