Libya's parliament has ordered the temporary closure of southern borders and declared seven southern regions restricted military areas.
A parliament spokesman, Omar Humidan, said the move was aimed at stemming the flow of illegal immigrants and goods.
There was no indication of how long borders with Chad, Niger, Sudan and Algeria would remain shut.
Libya's southern regions have struggled with lawlessness since former leader Muammar Gaddafi was toppled last year.
One member of parliament, Suad Ganur, said the situation had deteriorated recently because of possible international military action against Islamist militants in northern Mali.
She also told AFP news agency that there had been an "upsurge in violence and drug trafficking, and the presence of armed groups that act with complete impunity".
The move comes after the European Union proposed to help train Libyans to secure their southern borders and prevent the trafficking of arms from the country.
The parliamentary decree said the southern regions of Ghadames, Ghat, Obari, Al-Shati, Sebha, Murzuq and Kufra would be "considered as closed military zones to be ruled under emergency law".
The BBC's Rana Jawad in Tripoli says it is unclear what this latest decree means in practice because - in the continued absence of a conventional national army - most provinces and cities effectively rule themselves.
There was little sign of any change after the government declared a military area in the mountains near the western city of Zintan earlier this year, our correspondent says.
Also on Sunday, gunmen killed four policemen in an attack against a police station in the eastern city of Benghazi.
Officials said they thought the attack was linked to the detention of men suspected of involvement in a series of recent assassinations.
At least two members of the security forces were killed in a separate incident in the town of Bani Walid, a was a stronghold for Gaddafi loyalists during the uprising against his rule.