MIAMI — The new cap plugging the gusher in the Gulf of Mexico was holding up on Saturday but BP officials still weren't sure if and when they would begin siphoning oil to containment ships on the surface.

A 48-hour test to make sure the well could withstand the pressure from the cap was supposed to end on Saturday afternoon, said BP Senior Vice President Kent Wells.

But, ``the longer the tests go, the more confidence we have in it.''

The well did not seem to be leaking, however it was still too soon to tell what the next step would be.

``We're encouraged at this point,'' Wells said.

Meanwhile, BP continued to make progress on the first of two relief wells. The kill procedure could take place as early as August.

At the site of the gusher, BP measured temperature and seismic activity.

Company executives were particularly concerned with the pressure readings. A reading of over 8,000 psi, or pounds per square inch, would indicate the well casing was intact.

On Saturday, pressure in the well reached 6,745 psi and continued to rise by about 2 psi per hour.

The company also brought in ships to survey the area and ensure there were no other leaks.

If there is a leak, BP would have to open the valves holding back the oil, and allow the crude to once again flow freely into the Gulf. It would then have to hook up another riser pipe to a containment ship, which would likely take until late July.

It was not known when the tests would end.

BP did not plan to hold an afternoon conference call for the media on Saturday.

Said Wells: ``The test was set up to take 48 hours, but there was always the provision that under certain circumstances, the test could be extended. So far, with everything we've seen, there's no evidence that we don't have integrity.''