"The situation is very tense, things are finely balanced and the outcome is by no means certain," WTO spokesman Keith Rockwell told reporters as marathon talks stretched into their ninth day.
He added that delegates were focussing on the sticking point of import tariff changes.
India and the United States earlier accused each other of delaying progress towards an agreement on the issue.
The bargaining is aimed at securing consensus on measures to reduce barriers to global commerce under the so-called Doha Round, which has repeatedly foundered since it was launched in the Qatari capital seven years ago.
Talks since July 21 among ministers from about 35 key trading economies appeared to make a breakthrough on Friday, but optimism about a deal dimmed over the weekend as emerging economies held out for better terms.
"For everyone in the EU team there is a sense of being in the endgame," the European Union's Trade Commissioner Peter Mandelson wrote in Monday's edition of his daily bl