By Carol Giacomo, Diplomatic Correspondent

WASHINGTON, June 7 (Reuters) - Imposition of emergency rule would be a step backward for Pakistan and the United States hopes President Pervez Musharraf doesn't take such action, a senior U.S. official said on Thursday.

The official told Reuters the Bush administration was not aware of any plans by Musharraf to declare emergency rule but acknowledged fears among Pakistanis that this could happen.

"If he did that, it would be a significant step backward and ... of course we would not want it to happen," said the official, who spoke on condition of anonymity because of the sensitivity of the matter.

Musharraf, the frontline U.S. ally in the anti-terror war who is seeking re-election, triggered the biggest challenge to his authority since taking power in a bloodless coup in 1999 with his suspension three months ago of Pakistan's chief justice.

The crisis has led to speculation Musharraf might declare a state of emergency but he has repeatedly ruled that out and said elections due at the end of the year would be on time.

The United States is following the political turmoil in Pakistan intently but "I don't see anything right now that causes me great alarm" in terms of Musharraf possibly being overthrown, the U.S. official added.

Musharraf, who is also army chief, is due to address the nation, perhaps Thursday, Pakistani government officials said. He is expected to set out his position in a crisis he precipitated by suspending Chief Justice Iftikhar Chaudhry on March 9.

U.S. President George W. Bush and top aides have not openly expressed criticism of Musharraf's handling of the political crisis in his nuclear-armed state, even as U.S. analysts, major newspapers and even some officials voice new fears about the possibility of prolonged chaos or a coup.

Aiming to control the widening protests, the Pakistani government detained hundreds of opposition activists and a clamped-down on the broadcast media.

The U.S. official said the administration opposed the media crackdown, saying such tactics were "always counterproductive".

Musharraf repeated on Wednesday that elections expected later this year would go ahead as scheduled and be fair. But he is believed to be reluctant to give up his post of army chief as he is constitutionally required to do this year.

The U.S. official noted that Musharraf had said he would do nothing to circumvent the Pakistani constitution, but he declined to say whether Washington had specifically asked the Pakistani leader to reaffirm his promise to surrender the army post.

"We're always stressing the need to move ahead on the democratic transition" from military to civilian rule, the official said.

The administration has sought congressional approval to spend $31 million this year on democracy promotion in Pakistan, including election monitors.

U.S. Assistant Secretary of State Richard Boucher, who handles South Asia, is due to visit Pakistan next week and Pakistan's foreign minister, Khursheed Mehmood Kasuri, will be in Washington for talks June 18-20.

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