The report by the U.N. cultural and education agency UNESCO said good progress had been made toward meeting the education element of the Millennium Development Goals, a set of global pledges made in 2000 aimed at ending extreme poverty.

Primary school enrollment rose by 36 percent in sub-Saharan Africa and by 22 percent in South and West Asia from 1999 to 2005, spurred by the abolition of primary school tuition fees in 14 countries and other measures to promote attendance.

Worldwide, the number of out-of-school children fell from 96 million to 72 million in the period, UNESCO said. The proportion of children enrolled in school rose from 83 percent to 87 percent globally.

Public spending on education rose more than 5 percent annually in sub-Saharan Africa and South and West Asia, the regions with the most work to do to achieve the goals.

"We are steering the right course but as education systems expand, they face more complex and more specific challenges," said Koichiro Matsuura, director general of UNESCO.

Key challenges are to reach the most vulnerable and disadvantaged, to improve the quality of education as well as just boosting attendance, and increasing aid from rich states.

"At this midway point (toward 2015), our assessment leans towards the positive but much more remains to be done if the goals are to be met," said Nicholas Burnett, director of the study titled "Education for All Global Monitoring Report."

The report said an Education for All Development Index calculated for 129 countries where data was available showed that 25 states were far from achieving the goal. Two-thirds were in sub-Saharan Africa, and others at risk included Bangladesh, India, Nepal, Mauritania, Morocco and Pakistan.

The rise in primary school enrollment presents new challenges, the report said. Schools often relied on lesser-trained contract teachers to fill gaps. It said the world would need 18 million new primary school teachers by 2015.

Governments are also neglecting adult literacy, UNESCO said, with some 774 million adults worldwide, or one in five people, lacking basic literacy skills.

"Women's literacy in particular has a strong influence on a child's education and health yet they still account for 64 percent of adults who are not literate worldwide," it said.

UNESCO said external foreign aid for education was "far short of the $11 billion required annually," and was not targeted enough at sub-Saharan Africa or toward primary school education.