An undersea accident has forced BP to shut down a key system collecting oil from its blown-out Gulf of Mexico well, unleashing a torrent of oil, the top US oil spill official said.

A containment cap that had been channelling over 16,000 barrels per day to a surface vessel was removed after an undersea robot apparently collided with it, Coast Guard Admiral Thad Allen said at a news briefing in Washington.

He said the system could restart later on Wednesday local time.

Mr Allen said the flow of oil from the well was not completely unchecked. Some was still being burned off on the surface.

Mr Allen also reported two deaths of workers helping with the oil spill response. The deaths did not appear work-related but were under investigation, he said.

A team of US scientists estimates the leak is spewing up to 60,000 barrels a day.

The disaster prompted the Obama administration to slap a six-month ban on all new deepsea drilling while it sought to improve safety procedures on other rigs in the Gulf of Mexico.

While BP struggled to restart its oil collection operation, the US government said it would impose a more flexible ban on new deepwater drilling, a day after a federal judge overturned an initial moratorium as too broad.

Interior Secretary Ken Salazar also said initial investigations showed "reckless conduct" in the days leading up to the April 20 explosion on an offshore oil rig that ruptured BP's Gulf of Mexico well.

Mr Salazar told a congressional hearing in Washington he would reissue the drilling ban.

He did not indicate when he would issue the revised order, but suggested it could allow oil companies to drill in certain low-risk areas.

"We will in the weeks and months ahead take a look at how it is that the moratorium in place might be refined," he said.

He did not elaborate on his observation that he believed reckless behaviour was involved in the April 20 rig blast in which 11 workers were killed, and he did not point the finger at any particular company.

US politicians, however have accused BP of cutting corners and putting savings over safety. The company leased the rig from Transocean and was a part owner in the ruptured well with Anadarko.

The criticism has fuelled investor fears about BP's future and its stock has tumbled since the April 20 spill, losing half its value and trading at levels not seen since 1996.

In another problem for the British company, New York State's pension fund, a BP investor, has threatened to sue over the halving of its stock price since the start of the 65-day-old crisis.

The oil slick, which consists of hundreds of thousands of patches of crude, has shut down rich fishing grounds, killed hundreds of turtles and seabirds and dozens of dolphins. It has also soiled the coastline in four US states.

- Reuters