The defence ministry said in a statement that the soldier from the 11th Artillery Regiment "went missing while performing his official duties" at the border.

"The search is still ongoing," it said, adding that the soldier went missing on Thursday near the villages of Narewka and Siemianowka in eastern Poland.

Defence Minister Mariusz Blaszczak said the soldier "had serious problems with the law and had submitted his resignation from the army".

"He should never have been assigned to border service," he said.

Since the summer, Poland has sent thousands of soldiers to the border to help prevent large numbers of migrants from crossing over from Belarus, an authoritarian former Soviet republic.

The West accuses Belarus of engineering the migrant influx by letting in thousands of people from the Middle East and encouraging them to cross into the EU as a form of retaliation against EU sanctions.

Minsk has rejected the accusations.

Belarus's border guard service earlier on Friday said that a Polish soldier was seeking "political asylum" in Belarus because he did not agree with Poland's hardline position on the migrant crisis.

It said the soldier, who it named as Emil Czeczko, was stopped on Thursday near Tushemlya, which is on the Belarusian side of the border close to Narewka and Siemianowka.