Six EU member states have sent a letter to the bloc's executive warning against halting deportations of Afghan asylum seekers despite major advances of Taliban militants in their country, a minister from one of the signatories said.

"That regions of a country are not safe does not mean that each national of that country automatically is entitled to protection," Belgium's state secretary for asylum and migration, Sammy Mahdi, said on Twitter late on Monday.

He named Germany, Austria, the Netherlands, Denmark and Greece as other signatories of the letter to the European Commission.

The Taliban, fighting to reimpose strict Islamic law after their 2001 ouster from Kabul, have stepped up their campaign to defeat the government as foreign forces withdraw from the country, capturing a sixth provincial capital on Monday.

The issue is expected to come up at an online crisis meeting of EU domestic affairs ministers on Aug. 18, which was arranged mainly to discuss a surge of illegal border crossings from Belarus to EU member state Lithuania.

Poland and Latvia have also seen an increased flow of migrants from Belarus.

Mahdi said deportations of rejected asylum seekers from Afghanistan who are ordered to leave the country must remain possible.

"Afghans that, after a thorough and independent investigation, obviously do not need asylum, cannot remain in Belgium," he said.

The European Commission said it had received the letter from the six countries and would reply in due time.