A "sea change" is needed to achieve a global goal of ending child marriage by 2030, campaigners said ahead of a major meeting on Monday aimed at stopping the practice.

Some 12 million girls a year are married before the age of 18 with often devastating consequences for their health and education, and ending the practice by 2030 is among the United Nations Sustainable Development Goals.

Rates have fallen in recent years, but advocate Lakshmi Sundaram said "a complete sea change" was needed as new drivers such as climate change and rising conflicts threatened to undermine progress.

"It's a pretty ambitious target," said the executive director of campaign group Girls Not Brides, which is hosting the three-day meeting in the Malaysian capital.

"What we do need to see is a real step up from governments and donors ... it's their duty to protect their citizens and the girls," she told the Thomson Reuters Foundation.

About 25 million early marriages have been prevented in the last decade, the United Nations' children agency UNICEF says.

The biggest decline was in South Asia, where the risk of a girl marrying before her 18th birthday has fallen from 50 percent to 30 percent, according to UNICEF.

Poverty is often the key reason for child marriage, but protracted conflicts for example in Syria or extreme weathers in countries including Bangladesh, Mali and Niger have put more girls at risk, Sundaram said.

"It brings with it a whole set of new challenges on how best to support girls in those situations," she added.

Early marriage not only makes it more likely that girls will quit school, but it also increases the risks of exploitation, sexual violence, domestic abuse and death in childbirth, rights groups say.

The practice affects over 650 million women and girls today, UNICEF figures show. The agency also warned there will be another 150 million girls affected by 2030 if the practice is not stopped immediately.

"The key drivers of child marriage are not only poverty and a lack of access to education, but also prevailing gender and socio-cultural norms," said Sivananthi Thanenthiran, executive director of the Asian Pacific Resource and Research Centre for Women.

Improving gender equality would help stop the practice, she added.

About 500 delegates from over 70 countries are attending the meeting, including 17-year-old Hadiqa Bashir from Pakistan.

Bashir escaped an attempt by her family to marry her off when she was 11, and went on to set up an all-girl group, Girls United for Human Rights, that campaigns against early marriages.

"It's about changing perception of the people and the way they think," she told the Thomson Reuters Foundation, saying she hopes to find new inspiration from the meeting.