Without a breakthrough in the coming days, the World Trade Organisation's Doha negotiations risk further years of delay.

On Tuesday, the United States said it was ready to cut its annual ceiling on trade-distorting farm subsidies to $15 billion -- a level lower than its spending in seven of the last 10 years -- in order to kick-start the talks.

But India, Brazil and other big developing countries said Washington had to make deeper cuts before they would offer concessions of their own because the ceiling represented virtually double the level of U.S. farm subsidies last year.

"Within what is politically viable, $13 (billion) is close to reasonable," Brazilian Foreign Minister Celso Amorim said.

Kent Conrad, a Democratic senator from grain producer North Dakota, expressed concern that subsidies would be capped.

He said U.S. trade chief Susan Schwab appeared "to be negotiating against herself. She's certainly not negotiating in the interests of hard-working family farmers in North Dakota and elsewhere in the country".

Officials said talks on getting developing countries to open up their markets in manufactured goods were heated and trade officials said they were extending hotel reservations into next week with talks likely go beyond Saturday's planned end date.

Without a breakthrough on the core issue of farm and manufacturing trade by the August summer break, the Doha round risks being put on hold by the U.S. presidential elections and could remain that way for a couple of years.

Billed as a chance to bolster the global economy and help fight poverty, the round has struggled from crisis to crisis since it was launched in 2001.

Some supporters say its conclusion this year would still send a signal against protectionism as economies slow.

Schwab said Washington's offer on subsidies on Tuesday was "a major move" and developing countries now had to follow suit by opening up their economies and farm markets to more imports.

Developing countries complain that huge U.S. subsidies squeeze their farmers out of the market, reducing food supplies and contributing to a recent spike in global food prices. The European Union also faces calls to cut its farm import tariffs.