The UN and activist groups have called on Honduras to protect the only witness to last week’s murder of indigenous rights campaigner Berta Caceres.
Gustavo Castro Soto, a Mexican national, survived the attack by gunmen in the Honduran town of La Esperanza.
He was ordered to stay in Honduras, although Mexico and the UN said it was too dangerous for him.
Ms Caceres had led a campaign by the indigenous Lenca people against a dam project which threatened their lands.
“Gustavo should immediately be provided with effective protection and permitted to return to his country,” said Michel Frost, UN special rapporteur on human rights campaigners.
“It is high time that the government of Honduras addressed the flagrant impunity of the increased number of executions of human rights defenders in the country, especially targeting those who defend environmental and land rights,” he added.
More than 200 rights groups also called for the protection of Mr Soto and his family.
Ms Caceres was shot dead after two men entered her house on 3 March.
Her mother said Ms Caceres had been killed “because of her struggle” for indigenous and environmental rights.
Ms Caceres had told police she had received death threats.
The Honduran authorities have condemned the killing, stressing that Mr Soto will receive special protection.
High murder rates
Ms Caceres was one of the founders in 1993 of the National Council of Popular and Indigenous Organizations of Honduras (COPINH).
In 2015, she was awarded the Goldman Prize, one of the most prestigious awards for grass-roots environmentalism.
The judges said she had “rallied the indigenous Lenca people of Honduras and waged a grassroots campaign that successfully pressured the world’s largest dam builder to pull out of the Agua Zarca Dam”.
The dam would have flooded large areas of land and cut off the supply of water, food and medicine for hundreds of Lenca people.
Honduras has one of the highest murder rates in the world.