A day after BP closed off the flow of oil from its runaway well in the Gulf of Mexico, officials said the signs from a crucial test of the well’s condition were encouraging.

Pressure readings in the well rose significantly in the 24 hours after the valves were closed on a cap at the top of the well, an indication that the well was in good shape. But officials voiced caution, saying that they had expected that the pressure might rise even higher, and that the possibility of damage from the April 20 blowout could not yet be ruled out.

Another possibility, they said, is that the reservoir has been depleted by three months of gushing oil.

“This is generally good news,” Thad W. Allen, the retired Coast Guard admiral who is overseeing the spill response, said Friday afternoon, about 24 hours into what was expected to be at least a 48-hour test. “But we want to be careful not to do any harm or create a situation that could not be reversed.”