The United States said on Thursday it had asked Uzbekistan to join the multinational coalition it leads against Islamic State, saying Central Asia's most populous state was free to choose a way of contributing to the fight against the militant group.

"We have asked Uzbekistan ... to join the coalition," Daniel Rosenblum, U.S. deputy assistant secretary of state for Central Asia, told reporters during a visit to the Uzbek capital.

Uzbekistan, a mainly Muslim nation with a population of 31 million, has been a strategic NATO partner in post-Soviet Central Asia, assisting a U.S.-led war on the purist Taliban movement in neighboring Afghanistan.

The U.S.-led coalition hitting Islamic State forces in Syria and Iraq has a military component, apart from efforts to stop a flow of financing to Islamic State, Rosenblum said.

The coalition also gathers information about the movement of people across borders and has five or six other "lines of efforts," Rosenblum said.

"Uzbekistan or any other country can choose to contribute to one or more of those elements," he said.

Uzbek officials could not be reached for comment.

Uzbekistan, a gold and cotton producer whose 78-year-old strongman President Islam Karimov has been in power since Soviet days, has been criticized by Western governments and human rights bodies for clampdowns on dissent and basic freedoms.

The authorities have cited a need to prevent any advent of militant Islam in the volatile region for their tough methods.

While noting "closer ties and better sharing information between our security establishments", Rosenblum lauded Uzbekistan for progress in eliminating child labor in cotton fields, for which Uzbekistan had also been strongly criticized.

He said however that more needs to be done in eliminating involuntary adult labor in cotton production.