Lithuania has built the first stretches of a steel wall on its border with Belarus since migrants from the Middle East and other areas began entering from Belarus this year.
The European Union accuses Belarus of deliberately encouraging the migrants to enter EU states Poland, Lithuania, and Latvia via Belarusian territory as a way of putting pressure on the bloc.
Belarus has repeatedly denied this but its president, Alexander Lukashenko, has said his country would no longer stop the migrants since EU sanctions drain resources.
All three EU countries have put stretches of razor wire on the border to stop the migrants. Last week Lithuania began putting up the first stretches of the 11-foot-high steel fence, topped with razor wire.
Virgilijus Raugale is a border guard commander.
"It's probably impossible to build a totally impassable obstacle, so I think that this barrier too could be overcome. But that would take a long time, and we would be able to react."
Lithuania wants to build 500 kilometers (300 miles) of the wall by September next year, supplemented by coiled razor wire and video surveillance.
In August, Lithuania started sending almost all entering migrants back to Belarus. More than 5,600 migrants have been prevented from entering since then, the border guard said.
Poland's defense ministry, meanwhile, said on Thursday Belarusian soldiers had threatened to open fire on Polish troops just across the border,
in what it called an attempt to escalate a crisis over migrants along their common frontier.