WASHINGTON (AFP) - The United States (US) voiced concern on Monday after Vietnamese authorities prevented two activists from meeting a US official who was visiting Hanoi for talks on human rights.

US officials said that they had invited pro-democracy campaigner Pham Hong Son and human rights lawyer Nguyen Van Dai, who both spent time previously in prison, to take part in talks on Friday with State Department official Dan Baer.

"We are troubled that Vietnamese authorities reportedly prevented activists Nguyen Van Dai and Pham Hong Son from meeting with Deputy Assistant Secretary Baer as planned," State Department spokesman Patrick Ventrell told reporters.

Mr Ventrell said, however, that Mr Baer was able to meet in prison with one of Vietnam's most prominent dissidents, the outspoken Catholic priest Nguyen Van Ly.

Mr Baer, who focuses on human rights, held talks with Vietnamese officials in Hanoi on Friday as part of an annual dialogue between the two countries on the communist nation's human rights.

Mr Ventrell said that the two countries had "candid and constructive" talks on issues including freedom of religion, freedom of expression and prisoners, although US officials did not report any concrete outcomes.

Relations between the former war adversaries have warmed rapidly since they normalised diplomatic ties in 1995, but the US has said that Vietnam must improve its treatment of its citizens if it wants greater cooperation.

Critics in the US Congress and activist groups say that the frequent US calls over human rights have failed to produce change from Vietnam.