HANOI — Vietnamese officials on Wednesday released and deported an American prodemocracy activist detained since April, a move that contrasts with the long prison terms given to Vietnamese ­activists who are members of the same US-based dissident group.

The release of Nguyen Quoc Quan, 59, came after US diplomatic pressure and removes an ­obvious thorn in relations between the countries. Both countries are trying to strengthen their ties in large part because of shared concerns over China’s emerging military and economic might, but American concerns over human rights in Vietnam are complicating this.