American security officers were trying to negotiate by telephone today with a Filipino gunman holding 21 hostages for a special day in a bank building inside the U.S. Navy base at Subic Bay.

The gunman demanded a helicopter to fly him to an unspecified destination.

One of the hostages seized by the unidentified man yesterday afternoon was a woman known to be pregnant.

A spokesman at the base 50 miles northwest of Manila said electric power had been cut to the Prudential Bank and Trust Co. The one-story building was pitch black inside and had no air conditioning. Outside temperatures reaches 90 degrees this afternoon.

The Navy said it did not know if any Americans were among the hostages, but a Manila newspaper said they included some Filipino dependents of U.S. servicemen.

The New rejected several calls by the gunman for food. Officers said they refused to risk the capture of another hostage to deliver the food and the gunman had refused to send out a hostage to get it.

Navy spokesmen said the gunman's only demand since entering the bank at 2:12 p.m. local time (1:12 a.m., EST) yesterday was for a helicopter, but they indicated he has not said where he wanted to be taken.

Marines, members of the base's Filipino security force, and soldiers and police officers took up positions, around the bank.

The siege began after the gunman ordered bank manager to telephone a U.S. Marine guard that the bank, a private concession on the base, was being robbed. The bank's vaults were thought to contain about $133,000 in Philippine pesos.

A base spokeswoman said the gunman agreed several times this morning to surrender hostages in exchange for a helicopter.

He reneged each time the chopper arrived, however, because it was landing about 175 yards away, on a foot-ball field, and because navy officials ordered the pilot to keep his rotors whirring until hostages were set free.

Navy spokesmen said the man was armed with a 45-caliber pistol, which he apparently carried into he bank with him, and shotguns taken from two bank security guards with 24 rounds of ammunition.