"I do not exaggerate when I say that the Hong Kong ministerial meeting is on a knife-edge," EU Trade Commissioner Peter Mandelson said in notes for a speech.
The European Commission has been blamed by the United States and other key WTO members for an impasse in attempts to clear the way for a deal in Hong Kong.
But Brussels is also under pressure from France not to make further farm concessions.
"We signed up to a 'substantial improvement' in market access," Mandelson said, referring to previous commitments by the EU and its trading partners for the WTO talks. "We have to make an offer on this basis that can be both prudent and real."
The core WTO countries are due to resume talks on Friday by telephone. EU officials have said they are "number-crunching" to see what kind of new offer they can produce on tariffs.
Mandelson said the EU could not avoid action on agriculture and other areas "either economically and morally" in order to unlock the potential development gains of the WTO round.
The talks were launched in 2001 as a way of helping the world's poorest countries.
Europe has already reformed the bloc's Common Agricultural Policy by shifting most of the subsidies paid to farmers away from trade-distorting payments linked to crop production.
But the United States, Brazil and Australia, some of the most important WTO countries, are insisting that Brussels must go much further with its planned import tariff cuts.
"If we want the US to reform its own domestic subsidy regime - and if we want the Brazilians to cut industrial tariffs and open up on services - we have to move on agricultural tariffs, there's no other way," Mandelson said.
HONG KONG MEET MAY BE SCRAPPED: US
World trade talks scheduled for Hong Kong in December could be cancelled if the European Union fails to come up with an ambitious offer on cutting farm tariffs, a US trade official said on Monday.
"It's certainly a possibility," said the official, who spoke on condition of anonymity. "If the EU is not at the table the round is not going to be successful. So people are trying to look at other alternatives and other contingencies."