A burst pipeline in Alaska's North Slope has caused the Arctic region's worst oil spill, spreading more than 250,000 gallons of crude oil over an area used by caribou herds and prompting environmentalists again to question the Bush administration's drive for more oil exploration there.

The leak was first spotted by a British Petroleum worker 11 days ago, and was reported to have been plugged a few days later. Initial hopes expressed by BP that the spill was limited to a few tens of thousands of gallons proved to be over-optimistic. Alaska's Department of Environmental Conservation has steadily increased its estimate of the size of the spill, the latest estimate putting it at around 265,000 gallons.

The leak, whose cause is unknown, occurred in a remote part of the most sparsely populated state in the United States, and it remains to be seen what damage, if any, it has done to ecosystems. It does, however, give grist to groups who have challenged Washington's assertion that oil can be prospected and shipped while leaving only the gentlest of "footprints" on the landscape.

"This historic oil spill is a catastrophe for the environment," Natalie Brandon, of the Alaska Wilderness League, said in a statement. "Tone-deaf politicians in Congress should now stop trying to push for more drilling through sneaky manoeuvres ... The fact that the oil spill occurred in a caribou crossing area in Prudhoe Bay is a painful reminder of the reality of unchecked oil and gas development across Alaska's North Slope."

The biggest battle has been over the fate of the Arctic National Wildlife Refuge, also on the North Slope, which the White House wants to open up. The initiative, championed from the moment the Bush administration took office in 2001, has been consistently blocked by Congress but is periodically revived.

A second battle, meanwhile, is taking place in a previously untouched corner of the National Petroleum Reserve on the North Slope. The Bush administration has allowed oil companies to prospect for oil and gas in an area covering 389,000 acres. Environmental groups have responded with a federal lawsuit, filed last Friday, in which they contend that the Department of the Interior has violated the Endangered Species Act and other laws in an area noted for its flocks of migratory geese.

It is not just environmentalists who oppose the administration's plans. Several prominent energy analysts, as well as Washington politicians, argue that the likely yield in unexplored areas of the North Slope is not large enough to justify the intrusion.

Alaskan politicians and industry lobby groups are heavily in favour of expanding exploration as it would bring jobs and other benefits to the state economy. The Bush administration, meanwhile, argues that further domestic exploration is essential if the United States wants to decrease its dependence on oil and gas from the Middle East.

Accidents and leaks have periodically occurred on the North Slope, and along the trans-Alaska pipeline that takes crude from Prudhoe Bay across two mountain ranges to the port of Valdez on the shores of the North Pacific. Saboteurs blew up a section of pipeline shortly after it opened in the 1970s, starting a major spillage. A hunter accidentally fired into the pipeline five years ago, causing $7m (