Senegal has formally protested to The Gambia about the execution of two of its nationals on Sunday.
Prime Minister Abdoul Mbaye summoned The Gambia's ambassador Mass Axi Gey and demanded that the life of a third Senegalese death-row prisoner be spared.
Two Senegalese men were among nine prisoners executed on the orders of President Yahya Jammeh.
He has vowed to kill all 47 death-row inmates by mid-September.
The executions were the first in The Gambia in 27 years.
A Gambian national living in the Canadian city of Vancouver, Alhaji Sowe, told the BBC his family had learned from national radio that his brother, Alieu Bah, was executed.
"I was shocked, but I wasn't surprised because of the history of Yahya. That man is capable of doing anything, unfortunately," he told the BBC's Focus on Africa programme.
He said his family in The Gambia have not been officially notified of the execution and did not know the whereabouts of his brother's body.
"Everybody is scared," he said.
Mr Sowe said The Gambian authorities last allowed him to visit his brother in prison in 2006, for 10 minutes.
His brother was a former lieutenant in the army who was arrested and jailed in 1997 for plotting to oust Mr Jammeh, he added.
"Yahya was scared of him. He was a threat to Yahya and consequently he [Yahya] killed him because of that," Mr Sowe said.
The BBC's Mame Less Camara in Senegal's capital, Dakar, says Mr Mbaye warned the ambassador that relations between the two countries would take a turn for the worse if the remaining Senegalese on death row was executed.
The ambassador promised to take the message "up the chain of command", a Senegalese government official is quoted by the AFP news agency as saying.
Mr Mbaye summoned the ambassador to his office following a directive from President Macky Sall, who said he was dismayed and surprised that the executions took place without Senegal being informed through diplomatic channels.
Earlier, The Gambia's Interior Ministry said the prisoners were shot dead by firing squad on Sunday.
"By the middle of next month, all the death sentences would have been carried out to the letter; there is no way my government will allow 99% of the population to be held to ransom by criminals," President Jammeh said in a speech, which was broadcast on national television on 19 August, marking the Muslim festival of Eid.
It was not clear what crimes each of the nine prisoners were executed for.
Amnesty International and the European Union's foreign policy chief, Catherine Ashton, have urged The Gambia to stop the executions immediately.
Many of The Gambia's death row inmates are former officials and top military officers who have been detained for treason since 1994, when Mr Jammeh took power in a coup.
One woman was among those executed, the Interior Ministry said, listing crimes such as murder, treason, arson, drug and human trafficking as being punishable by death.
The death penalty was abolished when former President Dawda Jawara led the country but reinstated in 1995 shortly after Mr Jammeh seized power.