Homosexual acts are already illegal in Uganda. Under the proposed legislation, those convicted could face life imprisonment.

The law would also make it a crime not to report gay people.

In a statement, Mr Museveni said: "I... encourage the US government to help us by working with our scientists to study whether, indeed, there are people who are born homosexual.

"When that is proved, we can review this legislation."

Presidential spokesman Tamale Mirundi told Reuters news agency the bill would be on hold for now "until more conclusive research is done, and that's what the president is saying".

Mr Museveni originally refused to sign the bill, saying that it was wrong to punish people who were born "abnormal".

But then government officials said Ugandan scientists had advised him that homosexuality was a behavioural choice.

The scientists' report said there was no definitive gene for homosexuality but it was not an "abnormality" and it could be influenced by environmental factors.

"The practise needs regulation like any other human behaviour, especially to protect the vulnerable," it said.

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 that clause has been dropped.

Uganda already has legislation banning gay sex between men, but the proposed law sharply tightens restrictions - and covers lesbians for the first time.

Promotion of homosexuality - even talking about it without condemning it - would also be punishable by a prison sentence.