BP will monitor pressure readings from the new cap on the company's damaged oil well for an extra day after results so far showed no signs of any new leaks in the Gulf of Mexico.

The U.S. government added another 24 hours of monitoring as the 48-hour observation period expired Saturday.

Retired coast guard admiral Thad Allen, the U.S. government's point man on the disaster, decided Saturday that after testing is complete the well will be hooked up again to ships on the surface to collect the oil.

Although some oil would be released into the water temporarily to relieve pressure, it would not be at the same rate as previously.

Pressure readings from the new containment cap are slowly rising and there are no signs of any new leaks, the company said.

On Thursday, BP closed the vents on the cap and finally stopped crude from spewing into the Gulf — for the first time since the April 20 oil rig explosion that killed 11 workers and unleashed the spill about 1.6 kilometres below the surface.

Officials were worried because pressure readings were not rising as high as expected on Friday, indicating that oil may have been escaping.

But Kent Wells, a BP vice-president, said during a Saturday morning conference call that pressure inside the cap was slightly above the last level announced late Friday.

That level, recorded a day after the flow of oil was choked off, measured about 6,700 pounds per square inch. BP says it wants a pressure reading closer to 7,500 psi.

The new containment capping stack, in an image captured from a BP live video on Saturday. (BP handout/Reuters)

A possible new breach underground was a major concern going into the test, because oil breaking out of pipes in the bedrock would be harder to control and could endanger plans for a permanent plug.

The cap is designed to prevent oil from spilling into the Gulf, either by keeping it bottled up in the well or by capturing it and piping it to ships on the surface.

Either way, the cap is a temporary measure until a relief well can be completed and mud and cement can be pumped into the broken well deep underground to seal it more securely than the cap.

BP is drilling two relief wells, one of them as a backup. Wells said work on the first one was far enough along that they expect to reach the broken well's casing, or pipes, deep underground by late this month.

Then the job of jamming it with mud and cement could take "a number of days through a few weeks."

With files from The Associated Press