Pakistan appointed a former information minister and human rights campaigner as its ambassador to the United States on Wednesday, moving quickly to fill a post left vacant after tension between the civilian government and military.

The prime minister is pleased to appoint Sherry Rehman as the new ambassador to the United States, said the spokesman for the prime minister's office, Akram Shaheedi.

Rehman is veteran member of President Asif Ali Zardari's Pakistan People's Party and staunch proponent of civilian rule.

Former ambassador Husain Haqqani resigned on Tuesday, days after a Pakistani-American businessman accused him of being behind a memo that said the military was plotting a coup and appealed to the Pentagon to help ward it off.

Haqqani denied any connection with the memo.

The controversy has thrown a spotlight on the fundamental tension in Pakistani politics since the nation was founded in 1947 - competition for power between civilian politicians and military commanders.

The military has ruled the nuclear-armed South Asian country for more than half its history.

Rehman, a former journalist, was information minister for Zardari after the restoration of civilian rule following a 2008 election.

But she resigned in March 2009 over a crisis in which the government was refusing to reinstate judges sacked under the military regime of President Pervez Musharraf.

Her appointment was a surprise, with many analysts and media pundits expecting someone seen as closer to the military.

Rehman has been a strong advocate of women's and minority rights and faced death threats for her calls to reform the country's harsh blasphemy laws.

But she is also the author, through her role as president of the Jinnah Institute, of a report on Pakistan's foreign policy goals in Afghanistan, staking out views widely seen in Pakistan as dovetailing with those of the powerful military establishment.

(Writing by Chris Allbritton; Editing by Robert Birsel)