BRUSSELS—Six EU member states have warned the bloc’s executive against halting deportations of rejected Afghan asylum seekers arriving in Europe despite major advances of Taliban militants in their country. The Taliban, fighting to reimpose strict Islamic law after their 2001 ouster, have made sweeping gains in their campaign to defeat the government as U.S.-led foreign forces pull out. “Stopping returns sends the wrong signal and is likely to motivate even more Afghan citizens to leave their home for the EU,” Austria, Denmark, Belgium, the Netherlands, Greece, and Germany said in an Aug. 5 letter to the European Commission. “This is why we urge you and your teams at the Commission to intensify talks with the Afghan government on how returns to Afghanistan can and will continue in the coming months,” they said in the letter seen by Reuters. Many EU member states are nervous that developments in Afghanistan could trigger …