Pascal Lamy, head of the 149-nation World Trade Organisation, told an informal gathering of negotiators that "direct ministerial involvement" would be required in the final week of June, said officials.

The goal of the open-ended talks in Geneva would be to settle differences on what are known in WTO jargon as "modalities" -- the mathematical formulas for cutting barriers such as customs duties and subsidies.

These represent a key stepping stone, Lamy said, because they would "open the gateway" to concluding an international trade treaty after more than four years of stumbling talks.

WTO negotiators have missed a string of deadlines since the so-called Doha Round talks began.

Under an agreement in Hong Kong last December which aimed to steer the round to a conclusion within 12 months, negotiators were meant to have reached a deal on modalities by the end of April this year.

Lamy said that negotiators had to reduce enduring differences over trade in agricultural and industrial goods in "a matter of days, not weeks".

The two senior trade diplomats who chair the agricultural and industrial goods talks have for months been trying to prepare draft texts that would eventually be put before ministers, and officials said that they were likely to have them ready around June 19.

The farm and industrial goods trade have both proven major sticking points in the Doha Round, which was launched in the Qatari capital in 2001.

The goal of the round is to slash trade barriers and use commerce to boost the economies of developing countries.

Developing nations, led by Brazil and India, are pressing the European Union to offer deeper cuts in its agricultural customs duties than it has already put on the table, and are demanding that the United States slash susbidies paid to its farmers.

Brussels and Washington, meanwhile, are pushing developing countries to offer more open markets for trade in industrial goods.