NATO Secretary-General Jens Stoltenberg made the statement after discussing the border standoff with Polish President Andrzej Duda, Poland's PAP news agency reported.
Taking to Twitter after the talk, Stoltenberg said that "Belarus using migrants as a hybrid tactic is unacceptable."
He added: "NATO stands in solidarity with Poland and all our Allies in the region."
NATO on Monday condemned the use of migrants by Belarus "as a hybrid tactic" in a statement cited by Poland's PAP news agency.
The condemnation came as hundreds if not thousands of migrants gathered on the Polish-Belarusian border in an apparent bid to force their way into Poland and the European Union, the Polish state news agency reported.
"We are concerned about the recent escalation on the Polish-Belarusian border,” NATO said in a statement on Monday, as quoted by the Polish news agency.
“We urge Belarus to respect international law. We are witnessing a wave of migrants trying to get to the territory of the allies via Belarus," the statement said, according to PAP.
"NATO continues to closely monitor the situation that is putting pressure on our allies, Lithuania, Latvia and Poland," it added.
The Western military alliance was quoted as saying that "the use of migrants by the Lukashenko regime as a hybrid tactic is unacceptable."
It also said that Stoltenberg was in close contact with the governments of the allied countries, PAP reported.
Polish border guards, police and soldiers on Monday thwarted several attempts by migrants to force their way into the country via Belarus, government officials said, as the border crisis escalated.
The European Union has accused Belarus of encouraging thousands of migrants from the Middle East and Africa to cross into EU countries via Belarus, as a form of hybrid warfare in revenge for Western sanctions on Minsk over human rights abuses, the Reuters news agency reported.
Poland and the Baltic states have accused Belarus's strongman leader Alexander Lukashenko of organising a wave of illegal migrants seeking to enter the bloc as part of what officials have called a "hybrid war."