The new containment cap is pictured during testing in this image captured from a BP live video on Thursday. (Reuters)

Pressure readings from a new containment cap that stopped the flow of oil from BP's broken wellhead indicate there could be a leak down in the well, the U.S. government's point man on the disaster says.

Developments were "generally good news" but close monitoring must continue, retired coast guard admiral Thad Allen said on a conference call.

Engineers continue to look for evidence of leaks as pressure readings from the cap are examined, Allen said.

He said scientists believe the pressure levels could indicate that the reservoir — the source of the oil — is depleting after a three-month spill or that there's a leak somewhere down in the well.

"We don't know because we don't know the exact condition of the well bore," Allen said.

BP closed the choke valve on the new cap over its damaged wellhead off the Louisiana coast Thursday afternoon — stemming the flow of oil into the Gulf for the first time since the well blew out April 20 in an oil rig explosion.

Allen said the test will go ahead for another six-hour period before being reassessed to see if BP needs to reopen the cap and go back to piping some of the oil to ships on the surface.

The testing is to take place over 48 hours. Any new leaks would mean the cap would have to be reopened and oil would again be allowed to spill into the water.

If the testing proceeds as expected, it should wrap up Saturday afternoon.

Earlier, U.S. President Barack Obama said BP is making progress in stopping the flow of oil into the Gulf of Mexico, but a permanent solution is still needed.

Obama said Friday that scientists are monitoring the cap tests to ensure "we are taking a prudent course of action" and not simply looking for a short-term solution that could "lead to even greater problems down the road."

He said the placement of the new cap is "good news," noting that the new system will be used to stop the flow of oil or to capture much of the gushing oil until a relief well, which is seen as the more permanent solution to stopping the flow of oil, is finished.

"We're not going to know for certain which approach makes sense until additional data is in," he said, assuring the public that all decisions will be based on science and what is best for Gulf area residents.

Obama added that even if the cap can't completely shut off the flow of oil, it will allow BP to attach "many more containment mechanisms" so more oil can be channelled to ships on the surface instead of flowing into the Gulf.

Relief well essential

Kent Wells, a BP vice-president, said pressure is good inside the new cap and there were no signs of new leaks. He also said work would resume on one of two relief wells BP aims to drill.

U.S. President Barack Obama says the new cap is a sign of progress but cautions that a permanent solution is needed to stem the flow of oil into the Gulf of Mexico. (Susan Walsh/Associated Press)

Obama stressed the importance of the relief wells, saying: "We won't be done until we know that we've actually killed the well and have a permanent solution in place."

Though the flow of oil has been temporarily halted, Obama cautioned that it will be an enormous task to clean up the hundreds of millions of litres that have already poured out of the blown-out well.

With files from The Associated Press