US President Joe Biden has warned that Russia could be on the brink of invading Ukraine amid an apparent failure to withdraw troops from its side of the border and accusations of a possible false-flag operation in Ukraine’s disputed east. “We have reason to believe that they are engaged in a false-flag operation as an excuse to go in,” the US leader said on Thursday. “Every indication we have is they’re prepared to go into Ukraine.” The risk that Russia will invade Ukraine in the coming days is “very high,” Biden said. The Ukrainian government accused Russian-backed separatists of violating a …