Pakistani President Asif Ali Zardari is stable and resting in a Dubai hospital and will undergo further tests, according to his doctor, the presidential spokesman said on Thursday, hoping to quell speculation the unpopular leader might resign.

The president is stable, comfortable and is resting. Initial tests and investigations have been within normal range, spokesman Farhatullah Babar said in a text message.

Zardari is in Dubai for treatment for a heart condition.

Further tests will be carried out. The president is recuperating, said Babar.

One source said Zardari had suffered a minor heart attack.

His hospital stay has added a new element to uncertainty in Pakistan, an unstable, nuclear-armed U.S. ally facing everything from Taliban suicide bombings to crippling power cuts.

Babar later quoted the doctor as saying that Zardari had been moved out of the intensive care unit to the normal hospital room in Dubai where he is resting.

Zardari has been under heavy pressure in recent weeks following the resignation of the country's ambassador to Washington over an alleged memo to the Pentagon asking for help in forestalling a feared coup attempt in May.

The political saga is gripping Pakistan during a low-point in relations with the United States after a November 26 cross-border NATO air attack killed 24 Pakistani soldiers.

Washington wants to ensure Pakistan is stable so that it can play a major role in efforts to stabilise neighbouring Afghanistan.

The extent of Zardari's possible involvement in the memo case is a burning question in Pakistan, where the military dominates, setting security and foreign policy. Zardari had been due to address parliament this week after the Supreme Court admitted an opposition leader's petition demanding a judicial inquiry into the memo issue, including any role played by Zardari. That address has now been postponed.

CLINTON: NO REASON TO SPECULATE ABOUT ZARDARI

In Brussels, Secretary of State Hillary Clinton said the United States had no reason to speculate about Zardari's condition and expected him to be able to return to work in full health.

She was responding to reporters when asked about the possibility Zardari might be in the process of being pushed out for political reasons.

The information that ... we have is that he has sought medical treatment for a number of medical challenges and we wish him a speedy recovery and certainly we expect that...he will receive the treatment he is seeking and ... then be able to return in full health to his duties, she said.

Doctors overheard by a Reuters reporter in the hospital where Zardari is being treated, said the president had a fever and was being treated with antibiotics.

An official from Zardari's Pakistan People's Party said Zardari likely had a reaction to medications that aggravated a pre-existing cardiovascular condition.

The official ruled out the possibility that Zardari would resign and laughed off speculation that the military was trying to push Zardari out because of the memo issue.

Zardari, who has grown steadily unpopular since taking office in 2008, is looking more politically fragile after what has been dubbed memogate.

Zardari was married to late Pakistani Prime Minister Benazir Bhutto and came to power on the back of a sympathy vote after her assassination in 2007.

He has failed to gain the respect of many Pakistanis, and perhaps more importantly, the military.

Zardari, however, would become vulnerable to longstanding corruption charges in Pakistan by losing his legal immunity as a head of state if he steps down.

(Additional reporting by Chris Allbritton in ISLAMABAD, Praveen Menon in DUBAI and Arshad Mohammed in BRUSSELS; Writing by Michael Georgy, Editing by Jonathan Thatcher and Ron Popeski)