The deployment of two-thirds of Britain's active naval fleet to the area of the Falkland Islands has stripped Western Europe of most of its planned ocean defenses against a possible Soviet attack. But NATO officials here appear unconcerned--evidently because they consider a Soviet blitz attack unlikely.

Wartime plans for the North Atlantic Treaty Organization call for Britain to provide about 70 percent of the alliance's naval defense in the eastern Atlantic and to amass the forces within about a week of the outbreak of a military crisis.

The 20 British warships and 45 merchant vessels sent to the South Atlantic--about two-thirds of Britain's active naval fleet--are now more than two weeks' sailing time away from where the British Navy is supposed to be if Europe is attacked. With them are British Adm. John Fieldhouse, the allied commander-in-chief for the British Channel region, and the British Marine contingent designated to guard the approaches to Norway and Denmark in the event of war.

But senior NATO officials said today that barring an increase in East-West tensions, a temporary shift of British forces was not cause for worry.

"The lack is to be regretted," Adm. Robert Falls, chairman of NATO's military committee, said of the British fleet's absence from the North Atlantic. Speaking to reporters on the first day of a two-day defense ministers' meeting, Falls, a Canadian, said a "general feeling" existed among NATO nations that they should try to compensate for Britain's move by keeping their own forces "at a sufficient degree" of preparedness.

British Defense Secretary John Nott referred to the problem on April 7, just after the armada had sailed, when he acknowledged that the Falklands crisis would affect Britain's NATO commitment. "While the task force is deployed," he said, "clearly others of our friends will have to fill the gap left by our activities in that area."

Spokesmen for NATO's main land and sea commands, however, could cite no exceptional measures that have been taken--other than to amend alliance contingency plans--to offset Britain's venture to the South Atlantic.

A U.S. offer of material support to Britain was the subject of talks between Nott and Defense Secretary Caspar W. Weinberger. Nott said the talks had involved only "general terms" about "for instance, some logistic backup that might be desirable further on down the road."

Nott said Britain still had sufficient military forces and logistical support of its own "to carry our military mission to conclusion if necessary."

In a show of solidarity before today's full NATO conference, European defense ministers issued a statement this morning condemning Argentina's invasion of the Falklands and its failure to comply with U.N. resolution 502, which calls for Argentina's withdrawal from the islands and negotiations on the Falklands' future.

Nott said the statement "greatly heartened us," coming as it did after suggestions that the Europeans were cooling their backing for Britain following Britain's sinking Sunday of the Argentine cruiser General Belgrano.

Nott said he gained the impression here that Western Europe's support for Britain's cause "is as strong as it's ever been." He said he was confident that the European Community would, if necessary, vote to extend economic sanctions against Argentina, which expire on May 17.

In addition to the Navy and Marine forces with NATO defense roles, Britain has sent ground forces--the Fifth Infantry Brigade and two parachute battalions--not committed to alliance plans.

But even the shift of a considerable part of Britain's Navy did not appear to trouble NATO analysts. One NATO official said the idea that the Navy must be near Europe if war comes is somewhat artificial and stems from NATO's focus on its own limited boundaries.

If war came, this official said, it would probably be global and the exact location of the British Navy would matter less than the fact that it is already on combat alert.

Falls said NATO defense ministers received assessments today of the effect on NATO contingency plans of the British Navy's heavy presence in the South Atlantic.

Among the long-term effects NATO analysts expect from the fighting in the Falklands is a fanning of the debate over which land, sea, and air-based systems are most suited to modern warfare and should be funded first.

The fighting so far, some experts said, seemed to give both pluses and minuses for surface vessels--showing their ability to project force over a long distance as well as their apparent inability to evade increasingly sophisticated guided missiles.

On the other hand, it was a French-built Exocet that hit the British destroyer Sheffield--apparently proving that missile in action and putting a political premium on systems to defend against it and similar missiles, several NATO analysts said.