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UPDATE 2-BP test shows barely-rising pressure -Coast Guard

* Test to continue for another six hours -Coast Guard

* Coast Guard tells BP to step up monitoring of well (Updates with Coast Guard briefing)

HOUSTON, July 16 (Reuters) - A critical test showed pressure barely rising in BP Plc's BP.LBP.N blown-out oil well in the Gulf of Mexico on Friday, prompting officials to continue with more intense oversight.

“We have decided to move forward with another six-hour increment,” retired Coast Guard Admiral Thad Allen, the U.S. government’s point man on the spill, told reporters at a briefing on Friday afternoon.

The test is intended to show whether the April 20 blowout that preceded the leak damaged the piping and cement inside the stricken well, allowing oil and natural gas to leak out the sides and possibly breach the seabed.

BP shut off all flow of oil into the ocean on Thursday for the first time since the gusher burst in April to test well pressure. The shutoff was done with a new cap installed at the wellhead on Monday.

Allen had said pressure above 7,500 pounds per square inch would show the well is intact, while pressure that lingers below 6,000 psi would signal well damage.

He said on Friday afternoon that pressure remained close to 6,700 psi -- the same level as BP had announced eight hours earlier -- and was barely rising by two to 10 psi per hour.

As a result:

* Allen told BP to step up monitoring for any seabed breaches with underwater robots and gather additional seismic data to detect any pockets of oil in the layers of rock and sediment around the well.

* A nearby National Oceanic and Atmospheric Administration ship will use acoustic sensors to detect any gas bubbles that could signal a breach, Allen said.

* The more intensive monitoring should show if the pressure issue stems from a leak beneath the seabed or simply shows the reservoir has been somewhat depleted, Allen said.

* If BP and government scientists decide the risk of a leak is too high, BP will stop the test, open valves on a new cap atop the wellhead, allow oil to gush again into the Gulf, and restart a pair of oil-capture vessels to siphon some of the flow, Allen said. (Editing by Mohammad Zargham)