A large number of computers at a Geneva hotel that hosted delicate Iranian nuclear talks last year were infected with malware used for espionage, Swiss prosecutors said Thursday.

The Swiss Attorney General's office (OAG) however said it had closed its investigation, since it had failed to determine who was behind the spying.

Swiss investigators launched a probe in May last year based on "suspicion of illegal intelligence services operating in Switzerland," searching a hotel that hosted the nuclear talks and seizing computer equipment.

Those talks, which were held at a range of luxury hotels in Switzerland and Austria, concluded on July 14, 2015 with a landmark deal to rein in Iran's nuclear programme in exchange for the lifting of international sanctions.

The agreement between Tehran, Washington and five other major powers came into force in January.

Israel, which was vehemently opposed to the nuclear deal, was accused of espionage after a Russian-based security firm said a computer worm widely linked to the Jewish State was used to spy on the negotiations.

Israel flatly denied the accusations.

"Investigations revealed that a significant number of computers (servers and clients) at a hotel in Geneva had been infected with a form of malware," the OAG said in a statement Thursday, without divulging the name of the hotel.

"This malware was developed for the purposes of espionage, and is basically used to gather data from the computers infected," it said.

Investigators had however turned up "no evidence as to the identity of the perpetrators," it said.

"Accordingly, although there is evidence of criminal activity, it cannot be attributed to specific persons," OAG said, explaining why it had decided to close the case.