BANGKOK - It would be very hard for next year's elections in Myanmar to be legitimate without the involvement of detained opposition leader Aung San Suu Kyi, a senior United States official said on Thursday .

Deputy Assistant Secretary Scot Marciel, returning from a landmark two-day visit to the army-ruled country, said the release of the National League for Democracy (NLD) leader and other political detainees was critical for the polls to be considered fair and credible.

I think an election without Aung San Suu Kyi and the NLD, it would be very hard to see that as credible, Marciel told reporters in Bangkok.

The NLD, denied the chance to rule after a landslide win in the last elections in 1990, has yet to confirm whether it will participate in the polls.

In the end, it's up to Aung San Suu Kyi and the NLD to make that call, Marciel said.

Suu Kyi, a Nobel Peace Prize winner held captive for 14 of the last 20 years, was offered a rare chance to meet NLD committee members on Wednesday, but declined because its detained vice-chairman, Tin Oo, was excluded, state television reported.

Marciel declined comment on that development.

He stated repeatedly during a one-hour forum that Washington's objective was to encourage dialogue between the various camps inside Myanmar.

I frankly cannot see how there can be a credible election that brings legitimacy without inclusive participation and I don't see how that can happen without a dialogue, he said.

EXPLORATORY MISSION

The U.S. visit, the first of its kind in 14 years, comes as part of a new policy of engagement by the Obama administration and was described as an exploratory mission by Washington.

Marciel reiterated that the U.S. had no immediate plans to lift wide-ranging sanctions on Myanmar but said the embargoes would be reviewed, depending on reforms.

A policy that relies heavily on sanctions without dialogue has not succeeded but sanctions are still a useful tool, he said.

We do not think it is appropriate or wise to lift sanctions now in the absence of progress, but we certainly would be looking at them if there is progress ... The purpose of sanctions is to achieve an end.

The delegation met senior junta officials, ethnic groups, the NLD and Suu Kyi during the visit. Marciel did not elaborate on what was discussed, or why they failed to meet junta supremo Than Shwe, saying only: It's early in the process.

Marciel urged the military, which has ruled the former Burma since a 1962 coup, to be more inclusive and not to fear the prospect of democratic reform.

This is a country moving steadily backwards for a long time. There is a way ahead, but it will involve change and there cannot be progress without change, he said.

He urged the international community to judge Myanmar on results, not on pledges, adding the United States was taking a pragmatic approach and did not expect immediate progress.

We're going into this with eyes wide open, we're not under any illusions and we're aware that success is far from guaranteed, he said. We will proceed for a while and if it doesn't work, we'll try something new.

We should be very humble and not assume we have the answers until we have results.

(Editing by Alan Raybould and Jerry Norton)