Defence and security agencies were ordered to "close every metre of the border" in order to stop migrants that Lithuania turns away from coming back into Belarus.
Belarus President Alexander Lukashenko has ordered the country's borders to be tightened amid a row over migrants with neighbour Lithuania.
The Baltic country has seen a surge in migrant arrivals in recent months.
Vilnius claims it is being orchestrated by Lukashenko's administration as revenge for EU sanctions against Minsk, which were imposed following the diversion of a Ryanair plane to Minsk and the arrest of a dissident.
Earlier this week, Lithuania said it had turned away 180 migrants attempting to enter the country from Belarus.
On Thursday, Lukashenko ordered defence and security agencies to “close every metre of the border" in order not to let immigrants Lithuania turns away back into Belarus.
“God forbid they start implementing the policy of removing people they invited over there through official border crossing points,” Lukashenko said during a meeting with defence and security officials.
“Starting from today, not a single person should set foot on the territory of Belarus from the adjacent side, be it from the south or from the west.”
It comes after the Lithuanian Red Cross (LRS) said Vilnius' move to turn away migrants could be contrary to international law.
Speaking to the Associated Press, Egle Samuchovaite, programme director for LRS, stated that “pushbacks of people seeking asylum are not compatible with the Geneva Convention on Refugee Status, the EU Charter of Fundamental Rights and other human rights instruments”.
Rejecting the admission of vulnerable people into the country would put them into an unsafe environment, trapped between two countries, Samuchovaite said.
"In the absence of a physical border barrier with Belarus, the question arises as to how to ensure that there is no disproportionate use of force against asylum seekers, which by any means could not be justified," she said, adding that the Red Cross understands the state's challenges in protecting its border.
Lithuania, a nation of fewer than 3 million people, has no physical barriers for its 679-kilometre long border with Belarus. Some 4,090 migrants, most of them from Iraq, have crossed this year from Belarus into Lithuania.
Belarus claimed on Wednesday that a "non-Slavic" person died from injuries at a border town, but Lithuania dismissed the report as propaganda from a hostile regime.
"This is nonsense, a Brothers Grimm fairy tale," Lithuania Interior Minister Agne Bilotaite told reporters.
Defence Minister Arvydas Anusauskas called the report "an obvious provocation. Lithuania is under hybrid attack and spreading such information is a classic example of this process".
Later on Wednesday, the Belarusian State Border Committee claimed that five Iraqi migrants who were forcibly expelled to Belarus from Lithuania had injuries, including dog bites, and had been hospitalised.