United States President Barack Obama's administration said it would firmly oppose whaling, ahead of a key international meeting with pro-whaling Japan.
Anti-whaling campaigners said Mr Obama was signalling a tougher US stance leading into the meeting of the International Whaling Commission opening on Monday in Rome, which is set to look at a controversial compromise proposal.
Japan hunts hundreds of whales a year in the Pacific and Antarctic using a loophole in a 1986 IWC moratorium that allows "lethal research" on the ocean giants. Norway and Iceland defy the moratorium altogether.
"The United States continues to view the commercial whaling moratorium as a necessary conservation measure and believes that lethal scientific whaling is unnecessary in modern whale conservation management," said Nancy Sutley, chair of the White House's Council on Environmental Quality.
She said the Obama administration would wait and see the proposals on the table in Rome, but stressed, "It is our view that any package, to be acceptable, must result in a significant improvement in the conservation status of whales."
Japan has repeatedly threatened to leave the IWC if the 84-member body does not shift to what Tokyo believes is its original purpose - managing a sustainable kill of whales.
Japan's Antarctic whaling missions infuriate Australia and New Zealand and have been dogged by environmental militants, whose harassment has cut down the total catch.
Japan has said it will not halt research whaling, but is expected to make its own proposal in Rome that could reduce the number it kills.
Paul Kline, oceans campaigner for Greenpeace USA, said the environmental group was "thrilled" with the Obama stance. "It's great to see the United States putting out a strong position and positioning itself to truly be a world leader in whale conservation, which really supports the broad sentiment across America," he said.
"The past few years US leadership hasn't been there at all and the strongest voice has been taken over by Australia and a block of Latin countries led by Brazil."
Japan says whaling is a tradition and accuses Westerners of disrespecting its culture.