The US government said it had joined a number of developed and developing countries in filing a series of "collective requests" that press for more services liberalisation among World Trade Organisation members.

While the WTO's "Doha round" of negotiations has been focussed on agricultural tariffs, both the United States and Europe insist that services such as banking, computing and telecommunications must not be left behind.

Deputy US Trade Representative Susan Schwab said eight requests had been filed at WTO headquarters in Geneva, in advance of a special meeting devoted to the services chapter at the end of March and start of April.

She would not identify which other countries had joined the collective requests, but said the United States was taking part in all but one of the eight with the exception being maritime services.

"I think it's very clear that we're not going to have a successful outcome to the Doha Round unless agriculture, services and non-agricultural manufactured goods are all part of that equation," Schwab told reporters.

WTO members are striving to forge the outlines of a deal on agriculture by April 30, with a view to wrapping up the Doha round by the end of this year. But Schwab said the services portion of a deal would take longer than April.

"This is an aspect of the Doha negotiations that has largely gone on under the radar screen," she said, while emphasising that eight out of 10 US jobs are in services, in which the country enjoys a 56-billion-dollar trade surplus.

Among the services reforms sought by the United States are reductions to foreign investment limits, permission for banks and other operators to set up multiple branches and foreign managers to be allowed into closed sectors.

The US government quoted research from the University of Michigan that said services liberalisation could yield 1.4 trillion dollars in income gains for the world, or 72 percent of total gains from the Doha round.