Belarusian President Alexander Lukashenko has ordered security forces to tighten control over the border with Lithuania amid a dispute between the two countries.
Lithuania, an EU member, this week started turning away migrants attempting to cross from Belarus after facing a surge of mostly Iraqi migrants in the past few months.
The influx of migrants is seen by Lithuania as retaliation by Mr Lukashenko after the EU imposed sanctions on his country for diverting a Ryanair flight to the capital, Minsk, and arresting a dissident on board.
The surge of Iraqis and others is emerging as another source of tension between autocratic Belarus and its European neighbours to the west.
European Commissioner Ylva Johansson said on Monday that Belarus was cynically exploiting the hopes of Iraqi migrants by sending them on fruitless journeys.
While the new arrivals can apply for asylum in Lithuania, many are not thought to be refugees and will be returned home if their claims fail.
On Tuesday, Lithuania said it reserved the right to use force to stop such illegal immigration and turned away 180 people attempting to enter the country.
Mr Lukashenko ratcheted up his response on Thursday by ordering security agencies to “close every metre of the border” with Lithuania to prevent migrants being turned back to Belarus.
“God forbid they start implementing the policy of removing people they invited over there through official border crossing points,” he said.
“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.”
Authorities in Belarus this week alleged that Iraqi immigrants forcibly expelled from Lithuania to Belarus had injuries, including dog bites, and had to be admitted to hospital.
Belarus also 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.
Lithuania’s Interior Minister Agne Bilotaite called the report a “nonsense, a Brothers Grimm fairy tale”.
Defence Minister Arvydas Anusauskas said it was an “obvious provocation”. "Lithuania is under hybrid attack and spreading such information is a classic example of this process,” he said.
Lithuania, a nation of less than three million people, has no physical barriers on its 675-kilometre border with Belarus. About 4,090 migrants, most of them from Iraq, have crossed this year from Belarus into Lithuania.
The Lithuanian Interior Ministry this week distributed a video shot from a helicopter, showing large groups of immigrants being escorted to Lithuania’s border by Belarusian border guard vehicles.
Polish media report that some migrants have sought to enter EU member Poland from Belarus, although on a lesser scale.
The Belarusian State Border Committee said on Thursday that Lithuania “continues to force migrants to trespass the Belarusian border” and reported an attempt to “aggressively remove eight migrants … to the Belarusian territory”.
The border dispute comes as Belarus faces intense scrutiny over a series of international incidents.
Belarus Olympic sprinter Krystsina Tsimanouskaya this week decided to defect as she was being driven to a Tokyo airport after her grandmother told her it was not safe to return home.
The 24-year-old athlete, who was granted a humanitarian visa by Poland, caused a furore on Sunday when she said coaches angry at her criticism had ordered her to fly home from Tokyo.
After seeking protection from Japanese police, she flew on Wednesday to Poland instead of Belarus.
“Grandmother called me when they were already driving me to the airport," she said.
"Literally, I had some 10 seconds. She called me, all that she told me was: 'Please do not come back to Belarus, it's not safe'.
“That's it, she hung up. I would want to return to Belarus. I love my country. I did not betray it and I hope I will be able to return."
In another incident, Ukranian police opened a murder investigation after a Belarusian dissident was found dead in the capital Kiev.
Vitaly Shishov, 26, was discovered hanged in a park near his home in Kiev on Tuesday.
Detectives said they were pursuing several lines of inquiry, including “murder disguised as a suicide”.
On Wednesday, UK Prime Minister Boris Johnson increased pressure on Mr Lukashenko by pledging his full support for opposition leader Sviatlana Tsikhanouskaya.
Mr Tsikhanouskaya last year lost an election to Mr Lukashenko in a vote decried by western leaders as illegitimate.
