The 16 mostly Asian countries negotiating what will be the world's largest free trade area might not be able to achieve their target of a substantive deal by the year's end, an official source familiar with the talks has said.

Leaders of the countries participating in the Regional Comprehensive Economic Partnership free trade pact, who will meet in Singapore later this month for their summit, are likely to say that they have made "substantive progress" rather than saying they have achieved their target of substantial agreement, the source said Friday.

The biggest obstacle is the lowering of tariffs for goods, with India reluctant to offer anything that is politically sensitive as the country will be going through an election next year, the source said.