GENEVA (AP) _ Former Presidents Ronald Reagan and Gerald Ford today joined ex-British Prime Minister Margaret Thatcher in warning that the fighting in Bosnia could spread far beyond that country's borders.

''We are now witnessing in Bosnia a replay of one of the darkest eras of modern history: the invasion of one sovereign nation by another,'' said a statement issued by the former leaders.

The statement, distributed by Prince Sadruddin Aga Khan, former U.N. High Commissioner for Refugees, called the killing in Bosnia ''the attempted genocide of people who have lived in peace and tolerance with their neighbors for centuries.''

Human suffering from the widespread atrocities is appalling, it said.

''The savagery can and must be stopped or the tragedy will spread far beyond Bosnia,'' the statement said. ''People of conscience must speak out now.''

The former leaders urged more aid to Bosnian refugees to avoid deaths of ''countless more.''

Also signing the statement were former Secretary of State Alexander Haig, Jr., and Jeane Kirkpatrick, former U.S. ambassador to the United Nations.

The statement was put together on the initiative of Sadruddin, who had contacted the former leaders, said a spokeswoman.