HANOI (Reuters) - A senior U.S. diplomat told Vietnamese officials on Monday 12 April he was concerned human rights issues could impede the development of bilateral economic and trade ties.

There may be "implications" for the growing economic relationship between the former foes if human rights and labour issues are not managed properly, Robert Hormats, Under Secretary of State for Economic, Energy and Agricultural Affairs, told reporters without elaborating.

Between October and February Vietnam jailed 16 people in what Western diplomats and political analysts characterised as an unusually harsh crackdown on dissent in the Communist-run one-party state.

Human rights issues "are seen as important by a number of people in our country and therefore they could have an influence over a period of time on our economic relationship with Vietnam", Hormats said.

He said that in meetings with Vietnamese officials he voiced "concerns that, in general, if there were these issues, they could make it more difficult for us to proceed to do the kind of improvements in economic ties that we want to".

The goal was to resolve such issues constructively, he said.

Perceived backsliding on human rights has raised questions about whether Vietnam should be returned to the State Department's list of "Countries of Particular Concern" for religious freedom, which could carry economic sanctions.

Vietnam was on the list between 2004 and 2006.

The United States was the biggest single foreign investor in Vietnam last year, and also its largest export market.

Hormats said Vietnam was a "very strong partner" in Asia, a region where he said the United States wants to strengthen its role. Vietnamese Prime Minister Nguyen Tan Dung was in Washington for U.S. President Barack Obama's Nuclear Security Summit.

(Reporting by John Ruwitch; Editing by Jerry Norton)