BP chief executive Tony Hayward speaks at a press conference on the US gulf coast

BP has said it has drilled near its Macondo well beneath the seafloor in the Gulf of Mexico and may begin efforts in a few weeks to plug a leak that caused the biggest oil spill in US history.

The first of two relief wells detected the metal casing of the leaking wellbore at a depth of 5,050m and aims to intercept Macondo after additional tests and more drilling, BP said in a statement. An electromagnetic sensor found the leaking well, which is gushing as much as 60,000 barrels of oil a day into the ocean.

"Subsequent ranging runs will be needed to more precisely locate the well," BP said. "Drilling and ranging operations will continue over the next few weeks towards the target intercept depth of approximately 18,000 feet, when kill operations are expected to begin."

Those operations will include stanching the flow of oil with drilling mud and plugging the well with cement. BP has lost more than half of its market value since a 20 April explosion on the Deepwater Horizon drilling rig killed 11 crew members and triggered the spill.

BP failed in several efforts to stop the leak from above the well, including a so-called top-kill technique and placement of a 40ft-tall steel chamber that clogged with slush-like gas hydrates. The company is capturing some leaking oil through two systems that are taking crude to vessels on the Gulf's surface.

BP shares fell to their lowest since August 1996 last week in London. The company also dropped to record lows as the costs of the spill response reached $2.35bn (€1.9bn). Costs to protect against default on BP debt rose to an all-time high.

The first tropical storm of the Atlantic hurricane season was forming this weekend and may head into the Gulf, the US National Weather Service said on its website.

That threatens to stall drilling of the relief wells and cleanup efforts, which involve 4,500 vessels and 37,000 people.

US government forecasters say the hurricane season that began on 1 June may be the worst since 2005, when storms including Katrina devastated the US gulf coast and damaged offshore platforms and pipelines.

BP agreed this month to set aside $20bn in an independently managed account to cover cleanup costs and compensation claims from people affected by the spill.