Harvard international relations experts and diplomats yesterday said the Falkland Islands crisis could easily become a localized military confrontation because of nationalistic pride and the difficulty the United States may have being an impartial mediator.

Argentina invaded the Falklands last week, expelled the British governor general and about 100 British marines and established a military government. The islands, located 250 miles off the southern tip of Argentina, had been a British colony for 150 years.

Britain said Wednesday it will begin a blockade of the Falklands next week. Britain's Secretary John Nott declared English ships will sink any Argentine vessels within 200 miles of the archipelago. And Secretary of State Alexander Haig flew to London yesterday to confer with the British government and will go to Argentina this weekend.

Domestic Concerns

Nationalism and the domestic political concerns of the two governments are the main hurdles which could prevent a diplomatic settlement, the experts said. "The British, who are normally understated, are passionately nationalistic, especially in this time of their declining influence abroad," said Samuel H. Beer, professor of Government.

The experts agreed the government of Prime Minister Margaret Thatcher, already shaken by domestic political troubles stemming primarily from a slumping economy, could not afford to look weak in the international arena.

"There is little doubt that England is not going to settle unless Argentina makes very visible concessions. The fact that there seems to be much public sentiment in England for action makes the situation potentially very dangerous," said John S. Odell, professor of Government.

Beer explained that the Argentinian government also has internal political interests in keeping the Falklands. "There is some talk about oil and other material advantages on the Falklands, but the real issue seems to be symbolic." Beer said "General Galtieri probably wants to shore up popular support for his shaky regime."

The United States has not publicly supported either side in the conflict, through it did vote for a United Nations resolution condemning the Argentine take-over.

A spokesman for the British embassy in Washington said Haig is not an official mediator between the two countries but added. "Obviously, we are willing and happy to listen to any suggestions as to how this crisis can be peacefully resolved."

However, both experts and diplomats doubted that the United States could remain neutral if the conflict persists.

A State Department official said that Washington is "very concerned about the dispute. Both Britain and Argentina are our allies, and we would like to avoid offending either one. This might not he so easy," because of the uncompromising stances which each nation has taken.

"Right now," said Beer, "the U.S. is taking a middle road. But it would be ludicrous to think we will place more importance on the Argentine connection than on relations with Britain."

The other experts agreed that if the U.S. is forced to take sides, it will favor Britain.

"The move by Argentina is such a flagrant violation of international law, it is hard to imagine the U.S. siding up against Britain," Odell sai