BRUSSELS (Reuters) - The global sowing of genetically modified (GM) crops will continue rising in the next few years, gaining more of foothold in the world's food supply, but millions still need convincing that the food is safe to eat.

For once, green groups can agree with the biotech industry on one thing: with Brazil and China now part of the growing family major GM producers, the area of land devoted to gene-spliced crops across the world must inevitably rise.

The United States, Argentina, Canada and China are the world's leading growers of biotech crops. More than half of China's cotton crop, for example, is now genetically modified.

But there are doubts about how far the expansion can go, with questions lingering on China's commitment to GM crops and whether famine-hit Third World nations really want GM food aid.

In 2003, six countries grew 99 percent of the world's transgenic crop area, according to ISAAA, a non-profit group that backs biotechnology's role in the war on hunger.

"In the next five years, biotech crops are expected to grow to 100 million hectares planted by 10 million farmers in 25 or more countries," said Clive James, ISAAA's chairman and founder.

Most of those using the technology would be small Third World farmers. Maize and cotton would drive the growth, with soy production likely to rise after Brazil's recent approval of herbicide-tolerant beans.

ISAAA's estimate for the global GM crop area in 2003 was 67.7 million hectares (167.3 million acres), 15 percent higher than in 2002.

"There is a need for more acreages of grain. That will come from Asia to some extent, but also Latin America, Africa and Australia possibly as well," said Christian Verschueren, director general of CropLife International, a Brussels-based network representing the plant science industry.

Australia, which does not regulate GMO use in animal feed, approved its first GM food crop, canola, in July. Commercial GM crops are blocked by short-term bans by state governments.

India approved three varieties of BT cotton in 2002 for commercial product and is conducting field trials for several crops including mustard, rice, potatoes and cauliflower.