Bahrain set a two-week ultimatum Thursday for the return of citizens fighting as jihadists abroad, saying they will be charged under the Gulf kingdom's anti-terror laws if they do not.
An interior ministry statement said Bahrainis "currently in conflict zones... on the pretext of jihad" must "return to the country within two weeks".
Those who do not come back "will be pursued under the law pertaining to the protection of society from terrorist acts", it said.
Among the penalties that could be imposed would be loss of nationality.
At the end of February, the ministry warned that the law currently providing up to five years in prison for its citizens fighting abroad, including in Syria, would be toughened.
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It said it would pursue those Bahrainis fighting abroad, encouraging others to do so or belonging to "extremist religious groups or those considered to be terrorist organisations".
Bahrain, headquarters of the US Navy's Fifth Fleet, follows in the footsteps of neighbouring Saudi Arabia, which said last month that its nationals faced up to 20 years in jail for fighting abroad or belonging to "terrorist groups."
Riyadh toughened its stance on March 7, listing the Muslim Brotherhood and two Syrian jihadist groups as terrorist organisations, and ordering Saudis fighting abroad to return home within 15 days or face prison.
The tiny kingdom of Bahrain has a Shiite Muslim majority population but is ruled by the Sunni Al-Khalifa dynasty.
It remains deeply divided since a quashed 2011 Shiite-led uprising, with persistent protests sparking clashes with police, scores of Shiites jailed on "terror" charges and reconciliation talks deadlocked.
Last year, authorities increased the penalties for those convicted of violence, introducing the death penalty or life sentences in certain cases.