'Premeditated murder'

India alleges Jadhav was "kidnapped" from Iran where he was on business after retiring from the Indian Navy. He was arrested by Pakistani authorities in Baluchistan on March 3 last year.

Indian authorities say they learned of his arrest through a press release nearly three weeks later and were refused consular access multiple times, in violation of the Vienna Convention.

International law

Under the protocols, all disputes arising out of the interpretation or application of the Convention -- which governs diplomatic relations between states -- come under the "compulsory jurisdiction" of the ICJ.

In its petition to the court, India requested that Pakistan's sentencing of Jadhav be declared illegal and Islamabad directed to "release the convicted Indian national" in the event Pakistan does not annul its domestic judgment in the wake of the ICJ's ruling.

Sartaj Aziz, foreign affairs adviser to Pakistan's Prime Minister, said the government was "examining this issue."

On Twitter, Pakistan Defense Minister Khawaja Asif said India's letter to the ICJ was an "attempt to divert attention (from) state sponsored terrorism" in Pakistan.

Jadhav had been "convicted of offenses against (national) security," Asif said.

History of spying

Espionage has long been a fraught subject between Pakistan and India.

In 2013, Sarabjit Singh, an Indian man sentenced to death for spying, died after being attacked by fellow inmates in a Pakistan jail.

More than 40 alleged Pakistani spies have been arrested in India since 2013, according to the government.

Meanwhile, Pakistan, which executed 87 people last year, is the world's fifth biggest executioner.