The Council of Europe warned Turkey on Monday that restoring capital punishment after an attempted coup would be incompatible with its membership of the pan-European human rights organisation.
"No member state can exercise capital punishment," said Council of Europe Secretary General Thorbjorn Jagland, quoted by a spokesman.
"This is an obligation," he added, stressing that Turkey had ratified human rights protocols which "abolish death penalty under all circumstances".
Turkish President Recep Tayyip Erdogan has vowed to wipe out the "virus" of the putschists after facing down last week's coup bid by elements of the military.
On Sunday Erdogan told supporters that Turkey could consider reintroducing the death penalty, which it had abolished in 2004 as part of its longstanding EU membership bid.
Signed in Strasbourg in 1983, protocol 6 of the European Convention on Human Rights, drafted by the Council of Europe, excludes all capital punishment except in time of war or imminent threat of war.
The convention's later protocol 13, dating from 2002, closes the time-of-war loophole, stating clearly that "no one shall be condemned to such penalty or executed."
Turkey joined the Council of Europe three months after it was inaugurated in 1949.
The death penalty was abolished in Turkey in 2004, as part of the country's bid to join the European Union. Its application has been moving at glacial pace.