Officials from the Organization for Security and Cooperation in Europe met separately with Azeri and Armenian officials to discuss strains on bilateral ties.
The OSCE expressed, in a statement from Paris, its "deep concern" and "regret" for a decision by the Azeri courts to issue a pardon to Ramil Safarov. Safarov is an Azeri military official who was serving a life term in prison for the murder of an Armenian officer in Budapest in 2004.
The delegates said the pardon was a blow to trust on both sides of the Azeri-Armenian conflict.
The BBC reports that Safarov was promoted to major, awarded pay lost since his arrest and was welcomed to Azerbaijan as a hero when he returned last week.
Safarov was accused of hacking Armenian soldier Gurgen Makarian to death in Budapest, where both men were taking English-language courses established there by NATO, the BBC said.
Skirmishes broke out between Azeri and Armenian forces in June over the disputed territory of Nagorno-Karabakh. Both sides have gone to war over the territory, which Azerbaijan claims as its own.
Armenian President Serzh Sargsyan had said that his country wasn't interested in further escalation of the conflict.