Iranian gunboats attacked a Spanish freighter and a Liberian tanker in the Persian Gulf Monday, and reports indicated that 54 seamen may have died in a weekend Iraqi air raid.

The latest attacks were the fifth and sixth since Friday, when Iran, in apparent retaliation for the resumption of Iraqi strikes against its oil lifeline, ended a five-week lull in assaults on gulf shipping.One of Monday's victims, the 10,168-ton Spanish refrigerator ship Iberian Reefer, was reported at dusk to be burning fiercely, hours after it was attacked in the Strait of Hormuz. Salvage tugboats were en route, gulf-based shipping executives said.

The maritime insurance firm, Lloyd's of London, said 37 ships had been damaged in attacks by both Iraq and Iran so far this year, more than twice as many as in the first two-and-a-half months of 1987.

Last year was the worst for shipping since the war began in September 1980.

A senior Bahraini government official, while praising the efforts of foreign navies to protect their own shipping, suggested it was time for a joint declaration against any interference with shipping in the gulf.

Bahraini Information Minister Tariq al-Moayyed told the U.S. television network NBC that there have been no incidents involving commercial vessels sailing the gulf under naval escort.

As to what more the foreign powers might do, he said:

Jointly they must probably get together, and try to reach a declaration by which it's made clear to every potential aggressor that they would not

allow interference with freedom of shipping in the gulf.

He described this as an interim step to what he called a third stage, which is complete safety of navigation in the gulf, a return to the condition we had about 10 years ago.

Norwegian shipping officials said they had received reports of 54 seamen missing and possibly killed in Saturday's Iraqi air raid on Iran's Kharg Island complex in the northern gulf.

If correct, it would be the highest casualty toll in any attack on shipping by either side in the gulf, where the so-called tanker war began in earnest four years ago.

Iraq had claimed devastating results from the raid on Kharg, the second this year. Gulf-based shipping executives later confirmed that at least two of Iran's fleet of shuttle tankers had been hit and severely damaged.

Christen Puntervold, spokesman for the Norwegian Shipping Association in

Oslo, said only four of 29 crewmen aboard the 253,837-ton Sanandaj were known to have survived, and all 29 aboard the second tanker, the 316,379-ton Avaj, were missing.

A Norwegian Foreign Ministry spokesman said all of the missing were presumed dead.

Both ships are owned by the Iran's National Iranian Tanker Co., but the Sanandaj's crew was hired by a Norwegian firm, Viking Engineering, and another Norwegian company, Scanact, has part interest in the Avaj.

Mr. Puntervold said a Swedish engineering officer was among the casualties. Most of the other crewmen were Iranians or Asians.

Gulf-based shipping executives said they could not verify the reports, but one, asking not to be named, said he would not rule out the possibility that Iran had taken crewmen off the two damaged tankers, hoping to discredit Iraq for causing civilian deaths.

The highest casualties in a previous gulf attack, according to shipping records, were the 21 crewmembers missing after an Iraqi air strike in December on the Susangird, an Iran-chartered Norwegian supertanker. The fate of those men remains unknown.

The Iraqi attacks are aimed at disrupting Iran's vital oil exports, the mainstay of its economy, and forcing Tehran to accept a U.N. peace proposal.

Iran retaliates on a ship-for-ship basis by striking at neutral ships,

mainly those sailing to and from Saudi Arabia and Kuwait.

In Monday's Iranian attacks, a gunboat blasted the 61,762-ton Liberian- registered Fumi with rocket-propelled grenades, setting its engine room on fire. The tanker was en route to Japan with a cargo of Saudi Arabian oil.

Hours later, the Spanish-flag Iberian Reefer was attacked in the same area. A fire aboard was extinguished by the crew and the vessel, which was empty when attacked, later proceeded to Fujairah on the Gulf of Oman for repairs.