WARSAW—Poland will build a fence along its border with Belarus, the defense minister said on Monday, to halt a flow of illegal immigrants the European Union says is being driven by Belarusian President Alexander Lukashenko in retaliation for EU sanctions. Poland and fellow EU states Lithuania and Latvia have reported sharp increases in illegal immigrants from countries such as Iraq and Afghanistan trying to cross their borders. The EU says Lukashenko is waging “hybrid warfare” with immigrants to exert pressure on the bloc. “A new 2.5-metre-[8.2-foot]-high solid fence will be built on the border with Belarus,” Polish Defense Minister Mariusz Blaszczak wrote on Twitter. “More soldiers will [also] be involved in helping the Border Guard.” Poland’s government has come under sharp criticism from human rights advocates over the plight of a group of illegal immigrants trapped for two weeks in the open between Polish and Belarusian border guards near the …