Image caption Development Driller III has been inching closer for four months with the relief well

BP has begun pumping cement into the damaged oil well in the Gulf of Mexico in an effort to finally seal it.

The cementing of the bottom of the well, likely to be finished on Saturday, will bring to an end a nearly five-month battle for the oil firm.

No oil has spilled into the Gulf since a temporary cap was placed on the well in mid-July.

The explosion of the Deepwater Horizon rig on 20 April killed 11 workers and caused a massive oil spill.

The cementing operation began after a relief well finally intersected the damaged well on Friday.

BP spokesman Daren Beaudo said pumping started at 1330 local time (1830 GMT) and was expected to take "a few hours".

After the cement sets, engineers expect the well to be completely sealed, he said.

BP said earlier that when cementing is finished, the relief well would also be plugged and sealed.

The relief well was drilled 2.5 miles (4km) through dirt and rock beneath the sea floor.

This final cementing will mean BP can leave the site and concentrate on dealing with the aftermath of the spill.

At the beginning of August, the US government announced that almost three-quarters of the oil spilled in the Gulf of Mexico had been cleaned up or broken down by natural forces.

The remaining quarter was thought to be "degrading quickly".

But more recent research noted an undersea plume of crude oil-based chemicals up to 200m high and 2km wide, extending 35km from the spill site.

Despite optimism about the clean-up, the damage to the local economy, wildlife and the ecosystem of the Gulf is hard to fully assess yet.

On Wednesday, the US government said it also wanted energy firms to dismantle hundreds of unused platforms to prevent potential spillages; some installations have been sitting idle for decades without inspection for leaks.

The new requirements are due to take effect in October.