Pakistan's powerful military pledged on Friday to continue supporting democracy, reiterating it was not planning a takeover as tensions grew over a controversial memo alleging an army plot to seize power.

At the same time, President Asif Ali Zardari's spokesman said the president had resumed duties after returning from medical treatment in Dubai and had no intention of leaving over the scandal, which has undermined the already deeply unpopular president.

An army statement quoted military chief General Ashfaq Kayani as telling troops the military will continue to support democracy in Pakistan and that any talk the army was planning to take over was speculation.

Many Pakistanis wonder whether Zardari can survive the crisis, and speculation has been growing that the powerful generals will try to oust him somehow. The tension is a worrying sign for the region and for Pakistan's uneasy relationship with its key ally, the United States.

There are several scenarios under which Zardari could be forced out. The military does not want to be seen interfering in politics.

But it could use its vast influence to isolate Zardari, or offer him an honourable exit by guaranteeing he will not face prosecution on long-standing corruption charges.

Alternatively, Supreme Court justices, seen by some as anti-Zardari, could move against him in the memo case.

The United States wants political stability in Pakistan so that Islamabad can help fight militancy and aid Western efforts to stabilise Afghanistan.

In Washington, State Department spokesman Mark Toner told reporters the United States supported the democratic process and the rule of law in Pakistan and that such issues were for the Pakistani people to resolve within their own political process.

Zardari, known for his resilience in the face of pressure, plans to stay in Pakistan, said his spokesman.

He is performing his usual work. There is no truth in the reports that the president will leave the country after the 27th of December. He is here in Pakistan and he has come to stay, Farhatullah Babar told Reuters.

Zardari is expected to address a rally on December 27 on the fourth anniversary of the assassination of his wife, former Prime Minister Benazir Bhutto.

On Friday, Zardari chaired a meeting with senior leaders of his Pakistan People's Party, and also met coalition partners.

Pakistan's top judge earlier moved to allay fears of a possible military coup as tensions rose.

There is no question of a takeover. Gone are the days when people used to get validation for unconstitutional steps from the courts, Supreme Court Chief Justice Iftikhar Chaudhry said.

The Supreme Court is looking into a petition demanding an inquiry into what has become known as memogate. Kayani, arguably the most powerful man in the country, has called for an investigation into who may have been behind the memo.

'SPECTRE HAUNTS PAKISTAN'

Newspaper editorials on Friday highlighted unease in the nuclear-armed South Asian nation, predicting a showdown between Zardari and his allies and the military, which is so influential it has been described as a state within a state.

A spectre is haunting Pakistan - the spectre of a clash between the army and the government that threatens to turn fatal, said an editorial in the News.

Businessman Mansoor Ijaz, writing in a column in the Financial Times on October 10, said a senior Pakistani diplomat had asked that a memo be delivered to the Pentagon with a plea for U.S. help to stave off a military coup in the days after the raid that killed al Qaeda leader Osama bin Laden in May.

Ijaz later identified the diplomat as Pakistan's ambassador to Washington, Husain Haqqani, a Zardari ally who denied involvement but resigned over the controversy.

The military faced unprecedented public criticism over the bin Laden raid, widely seen as a violation of sovereignty.

But many Pakistanis rallied around the army after a November 26 air attack by U.S. forces in Afghanistan mistakenly killed 24 Pakistani soldiers on the border. The memo has also helped boost the army's image at the expense of the government.

Later on Friday, a Pakistani official said a planned briefing by the head of U.S. Central Command, General James N. Mattis, to explain the incident to Pakistani leaders had been cancelled.

Toner, the State Department spokesman, said the briefing was postponed at the Pakistanis' request, but not cancelled.

They decided to postpone it ... As you well know, there is some internal political dynamics right now in Pakistan so they felt it was best to postpone it until a later date, he said.

Zardari's government has become increasingly unpopular since he took office in 2008. It has failed to tackle myriad problems, from crippling power cuts to suicide bombings and a struggling economy.

The army is fed up with Zardari and wants him out of office, although through legal means and without a repeat of the coups that are a hallmark of the country's 64 years of independence, military sources told Reuters on Thursday.

TEMPERS FLARE

Another military source said tensions must be defused.

Tempers are flaring, there is no doubt about that. However, there are efforts to pacify the situation as well. And I hope they work, as under the current scenario, it is fast becoming a recipe for a head-on collision, he told Reuters on Friday.

Dawn, one of the country's most respected newspapers, said talk that a coup was being planned was premature, but also noted the army has seized power before.

The army remains the arbiter of power and analysts say it has plenty of ways to pressure Zardari to step down, especially if a link is established between him and the memo, which sought the Pentagon's help in averting a feared coup.

In the past the army has asked Pakistani civilian leaders to resign and influenced judicial proceedings against them.

On Thursday, Prime Minister Yusuf Raza Gilani told parliament that any institution acting as a state within a state was unacceptable, a clear reference to the military.

Although his position is largely ceremonial, Zardari wields considerable influence as leader of the ruling party and his forced departure would be a humiliation for the civilian leadership and could throw the country into turmoil.

Pakistan's next parliamentary elections are not due until 2013, although some opposition parties have been calling for early polls. Presidents are elected by legislators.

(Additional reporting by Arshad Mohammed in Washington; Writing by Michael Georgy; Editing by Paul Tait and Ed Lane)