NABLUS, West Bank, May 21 (Reuters) - Masked gunmen shot dead a West Bank man previously jailed as a suspected spy for Israel, the second apparent vigilante killing of a suspected spy this month, Palestinian security sources said on Saturday.

The sources identified the victim as Omar Helwan, 32, of Beit Dajan village near Nablus and said he had previously been held in a Palestinian prison on charges of working as an Israeli agent. Palestinian police were investigating, they added.

On May 6, a 30-year-old Palestinian man that Palestinian Authority President Mahmoud Abbas's security services had sought to arrest on suspicion of being an Israeli informant was killed by gunmen outside the hub city of Ramallah.

Palestinian militants have a tradition of inflicting vigilante punishments on people accused of collaboration with the Jewish state. During a Palestinian uprising which began in 2000, militants publicly executed people accused of tipping off Israeli security forces about the whereabouts of wanted men.

Such killings have become rare in the West Bank since a bolstered police force, supervised by Abbas's Western-backed government, has largely restored law and order in recent years.

Under Western scrutiny, Abbas has withheld the presidential approval required by law for carrying out death sentences against Palestinians convicted of treason.

Ignoring him, rival Hamas Islamists who control the Gaza Strip executed an accused spy on May 4, underscoring their differences with Abbas's Fateh movement on the same day the two groups signed a power-sharing deal in Cairo.

Separately on Saturday, Israeli troops shot dead a Palestinian spotted approaching its boundary fence with Gaza. The army said he was suspected of trying to plant a bomb against Israeli patrols.

Gaza hospital officials retrieved the body of a 19-year-old man who was not claimed as a member by any Palestinian militant groups, leaving the possibility that he had been planning to cross from impoverished Gaza into Israel to search for work.

(Reporting by Ali Sawafta, Mohammed Assadi and Nidal al-Mughrabi; Writing by Dan Williams; Editing by Peter Graff)