Breaking with the president she hopes to succeed, Hillary Clinton expressed her opposition Tuesday to Arctic oil exploration, one day after the US government approved Shell's exploratory drilling in Alaskan waters.

"The Arctic is a unique treasure. Given what we know, it's not worth the risk of drilling," Clinton said in a post on Twitter.

President Barack Obama's Bureau of Safety and Environmental Enforcement on Monday announced final approval for Royal Dutch Shell for the drilling -- already under way with a temporary permit -- which is the first of its kind since Shell's 2012 Arctic project was plagued by problems.

Obama angered environmentalists in May by authorizing drilling projects in the Arctic, which is believed to hold vast reserves of oil and gas.

While Clinton has expressed support for Obama's power plant emission regulations, her statement on drilling marks one of her most high-profile breaks with Obama administration policy, as the Democratic frontrunner seeks the support of the "green" vote while attempting to lock in her party's nomination.

She is facing an unexpectedly robust challenge from liberal Senator Bernie Sanders, an avid environmental defender who in May wrote Obama urging against approval of Arctic drilling.

"At a time when our planet is warming due to climate change, the last thing our environment needs is more drilling," Sanders said Monday.

Sanders is also fiercely opposed to the Keystone XL oil pipeline from Canada.

Clinton, who as secretary of state presided over part of the approval process for the pipeline, has so far declined to say whether she would approve Keystone.

Clinton's anti-drilling announcement earned swift rebukes from her Republican rivals, with Jeb Bush calling her "wrong" on energy policy.

"Being more-anti energy than Obama is extreme," Bush tweeted to Clinton. "We should embrace energy revolution to lower prices & create US jobs."