The United States said Tuesday a Central Asian bloc dominated by Russia and China was unlikely to turn into a military alliance but expressed concern over Iran's potential membership.

The six-nation Shanghai Cooperation Organisation (SCO) -- comprising China, Russia, Kazakhstan, Kyrgyzstan, Tajikistan and Uzbekistan -- seemed to have returned to its original goals of mostly improving border security and promoting counterterrorism, said Assistant Secretary of State Richard Boucher.

Leaders of the organization three years ago called for a deadline to be set for the withdrawal of US forces from bases in Kyrgyzstan and Uzbekistan.

When asked about that deadline at a Congressional hearing Tuesday, Boucher said the group might have "wandered off in various directions" but "I would say it probably stabilized again -- back to the basics: border security, cross-border cooperation, customs and border procedures, common efforts against terrorism."

"I think to the extent the organization has done those things, it has contributed to better security and stability to countries involved," he said.

There were fears by Western strategists that the group's holding of unprecedented war games last year -- involving 4,000 troops mostly from Russia and China -- could make its agenda increasingly military-oriented and set the stage for a new Warsaw pact rivaling the North Atlantic Treaty Organization (NATO).

"It's not becoming, as we see it, a sort of military alliance, certainly not an organization marshaling capabilities, commanding capabilities, instructing countries what to do, how to do it," Boucher said to questions from Democratic lawmaker Eni Faleomavaega, who chaired the hearing on Central Asia.

"It's not a Warsaw pact," Boucher stressed.

He said however that Washington would not remain silent if there were signs the grouping was evolving in that direction.

"Whenever we see it heading in that direction -- big countries telling little countries what to do -- we tend to stand up for the little countries," he said.

The two main US-led coalition bases in Central Asia are at Karshi-Khanabad in Uzbekistan and Manas in Kyrgyzstan. They have each been used to support US-led operations in Afghanistan since 2001.

Boucher said that last year, when the SCO held its meeting in Kyrgyzstan, the government in Bishkek made it clear that the Manas airbase was a "bilateral issue" with the United States and NATO and "was not a matter for discussion.

"And it didn't become a matter for discussion."

On the prospect of US archrival Iran, which is an observer in SCO at present, becoming a member, Boucher said there were recent reports that Iran had already asked for membership.

"I'm not sure what the organization will do," he said.

Asked by Faleomavaega whether it was "positive" for Iran becoming a member of the grouping, Boucher said: "Not particularly."