Authorities in the former Soviet republic have not confirmed the executions, but Valentin Stefanovich of Belarussian human rights group Spring 96 said prison officials had informed lawyers for Andrei Zhuk and Vasily Yuzepchuk of their execution.
"In Europe, there is no place for the death penalty. The Belarussian authorities should be aware that there cannot be political dialogue without shared values," the leaders of three European parliamentary bodies said in a joint statement.
Long ostracised in the West for his suppression of dissent, Belarusian President Alexander Lukashenko is seeking to reduce economic dependence on Russia and has taken some steps to improve ties with the European Union and United States.
"The decision to execute Mr Zhuk and Mr Yuzepchuk was taken in blatant disregard of the fact that the UN Human Rights Committee was still considering individual applications on their cases," the declaration said.
It was released by Sinikka Hurskainen, Jacek Protasiewicz and Uta Zapf, who lead bodies dealing with Belarus for the Parliamentary Assembly of the Council of Europe, the European Parliament and the OSCE Parliamentary Assembly.
The Belarusian Interior Ministry declined to comment.
Belarus is the only country in Europe that carries out executions, according to Amnesty International. Some other countries, including Russia, have not formally abolished the death penalty but observe moratoriums.
Under Belarussian legislation, official notification of the executions could take months, Stefanovich told Reuters.
Yuzepchuk was sentenced to death last July for the murder of six elderly women. At trial, his lawyer contended that Yuzepchuk had been beaten into confessing.
Zhuk was also sentenced last July, after being convicted of armed assault and murdering a man and woman in February 2009.
Lukashenko has ruled with an iron hand since 1994, winning re-election in votes denounced by the West and his opponents at home as illegtimate. On Thursday evening, more than 1,000 opposition supporters gathered for an annual protest commorating the creation of a short-lived Belarusian state in 1918.
The protesters marched toward Minsk's central square, where they had been forbidden to gather, but dispersed when they were blocked by more than 100 police.
(EurActiv with Reuters.)
