About half of 451 North Korean defectors questioned in a survey endured physical violence at the hands of North Korean authorities, a rights group has said as leader Kim Jong Un prepared to meet U.S. President Donald Trump for a summit.

U.S. lawmakers called Kim the "leader of perhaps the world's most repressive regime" on Sunday, but analysts say that as in the leaders' first summit, in Singapore in June, human rights are unlikely to be addressed in their second.

Trump and Kim are due to meet in the Vietnamese capital, Hanoi, on Wednesday and Thursday, eight months after their historic Singapore summit, the first between a sitting U.S. president and a North Korean leader.