BELFAST, Northern Ireland -- This year has begun as one of the bloodiest in the last decade in Northern Ireland, and officials fear the situation will only worsen in coming months as the 20th anniversary of the deployment of British troops here approaches.

So far, the 1989 death toll stands at 19 from sectarian violence between Protestants, who favor continued union with Britain, and Roman Catholics, who want the British out and reunification with the independent Republic of Ireland. The latest victims were two British soldiers, one Catholic civilian and three Protestant civilians killed last week in attacks for which the Irish Republican Army claimed responsibility.

Already the largely Catholic, outlawed Irish Republican Army, which is believed to possess an extraordinary stockpile of weapons and explosives, has made it clear that it intends to step up its attacks both here and on the British mainland before the deployment anniversary in August. At the same time, illegal Protestant paramilitary groups have launched what has been called "one of the most concerted loyalist campaigns for years."

Typically, most of the victims of "the Troubles," as the violence is known here, have been noncombatants. The IRA acknowledged after last week's shootings in Coagh that although it had intended to kill one man as an alleged Protestant paramilitary member, two pensioners were slain "in the general confusion." Seven civilians also have been killed by Protestant groups.

"Although 1988 was, by any standards, a bad year for violence in Northern Ireland, security sources are almost unanimous in privately predicting that the prospects for 1989 are, if anything, even worse," David McKittrick, award-winning Northern Ireland correspondent for the Independent, wrote recently.

The gloom prevails despite some rare, albeit tentative, political contacts between the two sides. The biggest concern is renewal of the "horrible tit-for-tat cycle" of death that peaked here in the mid-1970s, a spokesman for the Royal Ulster Constabulary said.

Of the 2,730 people killed here since British troops were sent into action Aug. 15, 1969, 1,651 of them died in the particularly violent 1971-1976 period. A total of 1,235 of those were civilians.

Security Heightened

Amid the latest spate of killings, authorities have significantly heightened their security efforts. But a number of factors are working against them, they say.

For one thing, Protestant paramilitary groups are better armed than previously believed, according to the spokesman for the Royal Ulster Constabulary. Two recent seizures of arms by the authorities included such heavy weapons as a rocket launcher and parts of a missile, and other raids reportedly have uncovered dozens of homemade machine guns.

Among this year's victims of loyalist assassination squads was Patrick Finucane, a prominent attorney who frequently defended alleged IRA terrorists. Another victim, however, was a member of the Free Presbyterian Church of the Rev. Ian Paisley, a unionist leader. The Presbyterian was gunned down in an apparent case of mistaken identity.

Sinn Fein Spokesman

But the bigger threat remains the IRA, which "would obviously want to drive home the message of the failure of Britain's policy" on the occasion of the 20th anniversary, said Danny Morrison, spokesman for Sinn Fein, the legal party usually described as the IRA's political arm.

"They probably want to inflict heavy casualties on the British," he added. "Whether they can is another question."

The IRA's message, McKittrick said, is that "they haven't been beaten in 20 years, and they won't be. So Britain might just as well pull out."

Meanwhile, officials continue to worry about the IRA's ability to carry on a sustained campaign of violence both here and in England itself.

The captain of a freighter captured by the French last year with 150 tons of munitions aboard allegedly admitted to having earlier delivered an equal amount of weaponry to the IRA from Libya's Col. Moammar Kadafi. According to the constabulary spokesman, there is "no doubt" that the "very, very sizable" IRA munitions stockpile includes SAM-7 missiles and flame throwers.

'No Evidence' of SAMs

Morrison confirmed that a flame thrower was seized recently in an IRA cache in western Belfast although, he said, "there's no evidence that the IRA possesses SAMs."