The US has added its voice to international criticism of Japan's largest-ever whaling expedition.
Japan says the hunt will be too small to affect whale populations
The UK, Australia and New Zealand have already urged Japan to call off this year's hunt, which will target humpback whales for the first time in decades.
Japan says the hunt is for scientific purposes and that the number of whales to be killed is too small to have a major impact on populations.
US officials say non-lethal techniques could achieve the same research goals.
The Japanese whaling fleet set sail on its five-month mission from the southern port of Shimonoseki on Sunday.
It has instructions to kill up to 1,000 whales.
As well as up to 900 minke whales and 50 fin whales, it will kill up to 50 humpback whales for the first time since a moratorium was introduced in the mid-1960s.
The species had been hunted almost to extinction before the ban.
'Guise and deception'
It was especially important that the Japanese whalers not kill humpback and fin whales, said state department spokesman Sean McCormack.
"While recognising Japan's legal rights, under the Whaling Convention, to conduct this hunt, we note that non-lethal research techniques are available to provide almost all relevant data on whale populations," he said.
New Zealand's Prime Minister Helen Clark said it would be better if the whaling fleet had stayed at home.
Environmental groups say they will shadow the whaling ships
She criticised "the guise, the deception, the claim that it is scientific whaling when they want to take 1,000 whales".
Mrs Clark added that it would be difficult for New Zealand to offer help if any of the ships got into trouble at sea.
The Australian government has also expressed disapproval, saying it is "deeply disappointed" by the launch of the expedition.
Foreign Minister Alexander Downer said there was no evidence of Japan producing any data from its research.
He said he had asked to see the Japanese ambassador but he ruled out deploying military defence forces, saying that Australia would not go to war with Japan over the issue.
His comments followed a suggestion by the opposition Labor party that they would send the Australian navy to track the fleet if they were elected in the imminent elections.
Britain has said it is considering high-level diplomatic action to protest against the hunt.
A UK government spokeswoman said the humpback hunt was unnecessary and that it has "serious reservations as to its scientific value".
"We are committed to maintaining the moratorium on commercial whaling and will oppose all efforts by Japan to undermine this with so-called scientific whaling," she said.
The hunt has drawn strong opposition from environmental and conservation groups.
Greenpeace is hoping to locate the fleet in order to shoot video footage, but claims the ships have turned off their identification equipment, making them hard to find.
The more radical Sea Shepherd group said its activists will attempt to intercept the ships.
The expedition is scheduled to run until mid-April 2008.