Human rights watchdog Amnesty International has called on Pakistan to resolve hundreds of cases of enforced disappearances for which "no one has ever been held accountable".

"Disappearances are a tool of terror... if committed as part of a widespread or systematic attack, they constitute a crime against humanity," Amnesty said in a statement issued on Monday, calling on Pakistan to "take concrete steps to end impunity".

Pakistan has had a history of enforced disappearances over the past decade, mainly confined in the past to conflict zones near the Afghanistan border or to Balochistan, where separatists have been agitating for independence.

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However, in recent years, a growing number of such abductions have taken place brazenly in major urban centres such as Karachi, Lahore and even Islamabad.

Security agencies routinely deny being involved.

Last year, five social media activists who had been critical of the military as well as extremism were also disappeared, with their abductions sparking nationwide protests.

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Four were released within weeks, but the fate of the fifth remains unknown.

Many other people are believed to still be in custody. According to Amnesty, the United Nations (UN) has more than 700 such cases pending in Pakistan, while a state commission of inquiry into enforced disappearances lists hundreds of additional cases.

Victims include bloggers, journalists, students, peace activists and other human rights defenders.

Few punishments, Amnesty said, are "as cruel and deliberate... Families are plunged into a state of anguish, trying to keep the flame of hope alive while fearing the worst. They may be in this limbo for years."

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According to the non-governmental organisation, Pakistan has recently accepted UN recommendations that make enforced disappearances a crime but has refused to ratify an international convention protecting anyone from enforced disappearances.