World governments should do more to help the 12 million "hidden population" of stateless people often living without basic rights, the office of the UN High Commissioner for Refugees said Thursday.

"A stateless person is effectively someone who does not exist," Assistant High Commissioner for Protection Erika Feller told reporters.

"We assess there are around 12 million around the world who are stateless ... and it is probably an underestimate," she said, launching a drive to spur nations to ratify and implement existing international conventions to reduce statelessness.

"They are a hidden population," added Feller.

As stateless people are often denied basic rights, from going to school to opening a bank account, "they thereby go in the margins of society," Feller said.

"If they cannot make ends meet because they cannot get a job, they go out and find illegal ways to support themselves.

"They then become a law and order problem. It is in a state's interest to regularise the situation."

Statelessness occurs due to a slew of reasons - from the denial of citizenship as in the case of Kurds living in Saddam Hussein's Iraq to the consequences of state breakups like the former Yugoslavia as well as the "unfortunate result of laws," said Feller.

"There are around 200,000 children born to Brazilians abroad. The legislation up to 2007 said that children who were born abroad to Brazilian citizens would not be able to obtain Brazilian nationality," Feller said, adding that the government was addressing the issue.

"Another form of legislation which leaves people stateless is gender discrimination... A woman in (around 30) countries is not able to confer her nationality on a child."

Feller said she understood the difficulties involved in encouraging states to change their citizenship legislation.

"Citizenship goes to the heart of state sovereignty, it is something that states guard very very jealously: their right to determine who is a citizen and who should stay on their territory," she said.

The UNHCR will hold a ministerial meeting in December for the 50th anniversary of the 1961 convention on the reduction of statelessness, which has been ratified by only 38 countries.

"We would like states to come and pledge concretely to end statelessness in their country ... (and) to assist stateless persons ... so they have access to health services, to education, to registration, even to death certificates.

"You can die as a stateless person, and there is no record of your death, because you don't exist," Feller said.