Image caption US offshore oil regulator Michael Bromwich said aging oil infrastructure posed an environmental risk

The US is set to require oil companies to plug 3,500 non-producing Gulf of Mexico oil wells in an effort to prevent future leaks, officials say.

The interior department will also require companies to dismantle 650 unused oil and gas platforms.

Some installations have sat idle for decades without inspection for leaks.

The order comes five months after an explosion at a BP oil rig in the Gulf of Mexico caused one of the worst environmental disasters in US history.

In an order issued on Wednesday, the interior department said wells that had not been used in the past five years for exploration or oil production must be plugged and their pipelines and rig structures dismantled.

Hazards 'reduced'

"As infrastructure continues to age, the risk of damage increases," said Michael Bromwich, director of the Bureau of Ocean Energy Management, Regulation and Enforcement, in a statement.

He said the new requirements, scheduled to take effect in October, would "substantially reduce such hazards".

Meanwhile, the senior US government official overseeing the BP oil spill response said the blown-out well was to be permanently sealed by Sunday.

"We are within a 96-hour window of killing the well," retired Coast Guard Adm Thad Allen told reporters.

BP's Deepwater Horizon oil rig exploded in April, killing 11 people. An estimated 4.9m barrels of oil then leaked into the Gulf.