Energy giant BP has announced the successful tests on a system to siphon oil from a blown-out well, a move the company called an "important step" in containing a spill that threatens environmental disaster.

Crude oil has been gushing into the sea from a ruptured undersea well in the Gulf of Mexico about a mile under the water's surface, threatening an ecological and economic calamity along the US Gulf Coast.

BP has succeeded in inserting a tube into the leaking well and captured some oil and gas, according to a statement posted on a website maintained by BP and US government agencies.

The operation was stopped briefly when the tube became dislodged, the statement said, but technicians inspected the system and reinserted it successfully.

President Barack Obama's administration, which over the weekend also pressed BP to make clear its commitment to covering the spill's costs, was unimpressed.

"This technique is not a solution to the problem," Secretary of Homeland Security Janet Napolitano and Secretary of the Interior Ken Salazar said in a joint statement.

The spill began after an April 20 explosion on the Deepwater Horizon rig that killed 11 workers. It threatens to eclipse the 1989 Exxon Valdez spill off Alaska as the worst US ecological disaster.