The Vatican has defended Pope Francis after human-rights groups expressed disappointment he did not publicly acknowledge the plight of Rohingya Muslims during a visit to Burma.

About 164,000 Rohingya have flooded into Bangladesh since violence erupted in Burma on August 25, the UN's refugee agency said.

About 18,000 Rohingya Muslims have fled fresh violence in Burma and crossed into Bangladesh, with hundreds stranded in no man's land at the border, according to the International Organisation for Migration.