India on Monday issued a stern warning after Pakistan sentenced to death an Indian 'spy' caught last year on charges of espionage and waging war against Islamabad.

The Indian External Affairs Ministry said Kulbhushan Jadhav, whose family lives in Mumbai, was sentenced "without observing basic norms of law and justice" and if he was hanged, it would be "premeditated murder".

Pakistan said earlier that a Field General Court Martial awarded the capital punishment and Army Chief General Qamar Javed Bajwa had confirmed it.

A Pakistani statement described Jadhav, who allegedly used the alias Hussein Mubarak Patel, as an Indian Naval officer attached to the Research and Analysis Wing (RAW).

Jadhav was arrested on March 3, 2016, reportedly in Balochistan, for "involvement in espionage and sabotage activities against Pakistan".

"He confessed before a Magistrate and the court that he was tasked by RAW to plan, coordinate and organise espionage, sabotage activities aiming to destabilize and wage war against Pakistan by impeding the efforts of law enforcement agencies for restoring peace in Balochistan and Karachi," the statement said. 

Foreign Secretary S. Jaishankar handed Pakistan High Commissioner Abdul Basit a demarche warning that if Jadhav was hanged, "the government and people of India will regard it as a case of premeditated murder".

It accused Islamabad of abducting Jadhav from Iran and "his subsequent presence in Pakistan has never been explained credibly".

It said the Indian High Commission in Islamabad sought consular access to Jadhav 13 times between March 25, 2016 and March 31, 2017 but was not given permission to meet him.

"The proceedings that have led to the sentence against Jadhav are farcical in the absence of any credible evidence against him.

"It is significant that our High Commission was not even informed that Jadhav was being brought to trial.

"Senior Pakistani figures have themselves cast doubt about the adequacy of evidence. The claim ... that Jadhav was provided with a defending officer during the so-called trial is clearly absurd in the circumstances."

Indian officials say Jadhav had left the Navy and was engaged in business in Iran, which shares a long border with troubled Balochistan.

In New Delhi, Shiv Sena chief Uddhav Thackeray urged the Indian government to take "extreme steps if required" to save Jadhav.

According to Amnesty International, Pakistan is among the world's top five executioners and 87 executions were recorded in the country in 2016.

As news of Jadhav's sentencing spread, neighbours and friends trooped to the Silver Oak building in Hiranandani Gardens in Mumbai, some with handmade placards.

As a precaution, the police deployed half a dozen constables outside the premises. Some policemen visited the family living on the fifth floor.

Most neighbours and friends said Jadhav could never be involved in any activity he has been accused of.

Jadhav's father, a retired Mumbai Police officer, has consistently maintained that his son had been framed. But no family member was seen on Monday in public.

Pakistan defended the death sentence.

"You can't sponsor terrorism and then summon an ambassador to protest over the sentence of terrorists. Nothing matters more than national security," Pakistani envoy Basit said in New Delhi.

He said Pakistan had done no wrong in giving death sentence "to a terrorist".

In Islamabad, Defence Minister Khawaja Asif said the death sentence should serve as a warning to those engaged in terrorism in Pakistan.

"Those plotting against Pakistan will not be spared," Asif told Geo News.

He said Jadhav's confession was a public document. "He came with the approval of the Indian government… There is no doubt that India is fuelling terrorism in Pakistan."