Travellers to the United States will face tougher Covid-19 testing rules, while more countries tighten their borders amid uncertainty around the virulence of the Omicron variant and its ability to evade vaccine protection.

Japan and Hong Kong said they would expand travel curbs and Malaysia temporarily banned travellers from countries deemed at risk. Japan, which had already barred all new foreign entrants, reported its second case of the new variant on Wednesday.

Other countries braced for more cases: Australia said at least two people visited several locations in Sydney while likely infectious...