India emerged as a major new irritant on Thursday in Western efforts to isolate Iran, announcing that it was sending a large trade delegation there within weeks to exploit opportunities created by the American and European antinuclear sanctions that are increasingly disrupting Iran’s economy.

The trade delegation announcement coincided with new reports that India, an important consumer of Iranian oil, had eclipsed China for the first time as Iran’s No. 1 petroleum customer last month, subverting efforts by the United States to persuade other countries to find non-Iranian sources for their energy needs or risk onerous penalties under a new American sanctions law.

The announcement also came ahead of a planned visit to India by Herman Van Rompuy, the European Union president, who was quoted in an interview with The Times of India as saying that he intended to seek the Indian government’s help in pressing Iran to give up its nuclear program.

It was unclear whether Mr. Rompuy knew at the time of the interview that India’s commerce secretary, Rahul Khullar, was about to announce a big economic push into Iran that could serve to counteract the effects of the very sanctions Mr. Rompuy has helped to promote.