LONDON, DEC. 17 -- Iranian speedboats twice attacked a cargo vessel carrying sulfur in the Persian Gulf today, in the latest assault during what shipping sources here described as a sharp tit-for-tat escalation in the tanker war this month.

The attacks, which set fire to the Maldives-registered Island Transporter, took place as the U.S. guided-missile destroyer Chandler stood nearby. Under Pentagon rules, U.S. naval forces in the area protect only American-registered ships or others specifically put under U.S. protection.

News agencies in the gulf said that the Chandler had radioed first reports of the attack to regional port authorities, but the Navy termed "erroneous" early accounts that the U.S. vessel had helped rescue some of the 37 crew members aboard the Island Transporter. All crewmen were reported rescued by salvage tugs.

A CBS News helicopter filming the second attack on the blazing ship, seven hours later, was fired on by the Iranians, but was not hit.

Today's incidents brought to 19 the number of attacks against merchant shipping by Iranian or Iraqi forces this month, according to Lloyd's Maritime Information Services, who closely monitor shipping activity worldwide.

With two weeks of the month still to go, December's total already is nearing the record months of October and November, during each of which there were 21 attacks. According to Lloyd's, there have been 163 attacks this year, for a total of 421 since May 1981.

Sources here said that, rather than calming the tanker war, the increase in foreign warships this year in the gulf -- now estimated at about 80 western vessels -- appeared to coincide with the stepped-up attacks.

Analysts in the gulf noted that Tehran had been more successful than ever before in inflicting heavy damage on its shipping targets. Early this month, Iran attacked the Singapore-registered tanker Norman Atlantic with rocket-propelled grenades, sinking a merchant target for the first time in the war.

But half of this month's attacks came from Iraq, whose air-launched missiles have had far more devastating effect on their targets.

On Dec. 9 and 10, an Iraqi missile attack against the Iranian tanker Susangird brought the highest death toll yet in any incident involving merchant shipping.

An estimated 22 crew members, including the Norwegian captain, are missing and believed killed. The ship was part of Iran's shuttle service carrying oil out of the principal war zone to loading terminals near the mouth of the gulf at the Strait of Hormuz. Although owned by the Iranian National Oil Tanker Co., the Susangird was operated by a Singapore company and manned under contract with a Swedish-Norwegian concern.

Ole Holthe, of the Norwegian Shipowners Assoc., said today by telephone from Oslo that Tehran still had not confirmed the deaths. "We can only rely for the time being on information received inter alia from our embassy in Tehran," Holthe said. "It's not a Norwegian ship, and our interest only boils down to the fate of the Norwegian master."

But "there is every reason to believe that he had very, very little chance of surviving the inferno," Holthe said. "So, alas, for the crew," composed of Polish and Filipino seamen.

According to Lloyd's, the Susangird was first attacked by Iraqi jets on Dec. 9, as it was traveling in a shuttle convoy. Another attack 12 hours later, Lloyd's said, caused a much more serious fire.

The burning ship quickly was surrounded by a ring of flaming oil that had spread into the water, Holthe said. Although five crewmen were believed to have escaped, the remaining 22, including the captain, are thought to have been trapped.

Iran later towed the ship to Larak Island, and the delay in confirming the casualties was thought due at least in part to difficulty in identifying the bodies.

Most of the attacks this month seemed to alternate between the two warring countries. While Iraq, as usual, targeted Iranian shuttle vessels, Iran moved against any unprotected merchant ship believed to be carrying supplies on behalf of Iraq or any of its perceived allies along the gulf.

According to Lloyd's, the month began with three attacks by Iraqi jets against the Cypriot-registered vessels Anax, hit on Dec. 2, and Actinia, attacked twice and seriously damaged on Dec. 4. Both vessels were shuttling Iranian oil between Larak and Kharg islands.

Iranian speedboats, firing rocket-propelled grenades, hit back Dec. 6, with an attack that caused slight damage aboard the Danish tanker Estelle Maersk, en route from the Saudi port of Jubail, and the attack on the Norman Atlantic.

On Dec. 8, Iraqi jets attacked the Iranian-registered Alamoot, a shuttle tanker that was towed to Larak Island with extensive damage. On Dec. 9 and 10 came the Iraqi attack against the Susangird.

From Dec. 11 through 15, Iran launched five attacks against merchant shipping. On Dec. 16, the two sides traded attacks. An Iranian gunboat shelled the Greek tanker World Produce and Iraqi jets attacked the Iranian tanker Taftan.