The Swiss Embassy, representing Britain's interests, has complained to the Foreign Ministry that British citizens here are being threatened with death because Argentines cannot visit grave sites in the Falkland Islands of relatives killed in the war there.

Swiss Ambassador Jean Pierre Keusch said yesterday that he decided to seek guarantees from the ministry after Financial Times correspondent James Burns and Theodore Oliver of the London Daily Mail were targeted Thursday night for death if they did not leave the country in 24 hours.

The threat came in a phone call to the British interest section of the embassy from a man identifying himself as a member of the 2nd of April commando group. On April 2 of last year, Argentina seized the islands from Britain, sparking the Falklands conflict.

Burns said he had asked the Interior Ministry for police protection. Oliver, who is Irish, could not be reached for comment and reportedly left yesterday for Uruguay on orders from his superiors.

The caller said the action against Burns and Oliver would be taken in retaliation for a British decision to bar relatives of Argentine soldiers who died in the 74-day conflict from visiting the islands later this month. Britain said they could come only in a non-Argentine ship. The caller also said the group would begin attacking British-owned businesses, singling out the Anglo-Dutch Shell Petroleum Co. and British schools.

On Thursday James Neilson, editor of the Buenos Aires Herald, an independent daily of majority American ownership, said he was warned by the Argentine Anticommunist Alliance, a terrorist group from the late 1970s, that he had "48 hours to get out of the country."