Pakistani President Asif Ali Zardari is undergoing what his office said on Wednesday were routine medical tests in a Dubai hospital, but which another source said was treatment for a minor heart attack, fuelling rumours that he may resign.

Zardari's office said a news web report, which triggered much of the speculation, was untrue. Financial markets were unaffected by the rumours.

President Asif Ali Zardari is in a Dubai hospital for medical tests and check-up as planned, presidential spokesman Farhatullah Babar told Reuters.

Reports in some sections of the media speculating on the president's activities and engagements are speculative, imaginary and untrue.

But a Pakistani source in Dubai familiar with the president's condition told Reuters that Zardari had suffered a minor heart attack.

Two days ago, he had chest pain and decided to go to Dubai, the source said.

Six years ago, Zardari had also had a minor heart attack, the source said.

Since then, he has been on medication.

A Dubai-based member of Zardari's Pakistan People's Party, Mian Muneer Hans, said the president landed in Dubai around 7:30 p.m. on Tuesday.

He walked to his car in the airport and was not on any ambulance, said Hans, adding that he was accompanied by his doctor and petroleum minister Asim Hussain. Zardari was taken straight to the American Hospital in Dubai, said Hans.

He's taking rest in the hospital now. He may be there for two to three days, he added.

The hospital's chief executive officer Thomas Murray, contacted by Reuters, declined to comment on the reports.

Hans, however, said the medical visit was a routine check for his heart.

The rumours about his health and possible resignation swirled on Twitter and other social media.

Some elements blew up this to create unrest in the country, said Fauzia Wahab, a senior member of Zardari's Pakistan People's Party. His visit to Dubai and having a medical check up is perfectly normal.

Pakistan's civilian government has been under extreme pressure in recent weeks following the resignation of its ambassador to Washington over an alleged memo to the Pentagon asking for help in forestalling a feared coup attempt in May.

Tension between the government and military have bedevilled the nuclear-armed South Asian country for most of its existence, with the military ruling the country for more than half of its 64-year history after a series of coups.

Relations with the United States have been rocked by a year of bust-ups despite some $20 billion in security and economic aid to Pakistan since 2001, much of it in the form of reimbursements for assistance in fighting militants.

First there was the jailing of a CIA contractor for shooting dead two Pakistanis in the city of Lahore. Then there was the secret U.S. commando raid inside Pakistan that killed al Qaeda leader Osama bin Laden, and then came U.S. accusations that Pakistan was involved in attacks on American targets in Afghanistan.

It was also further rocked by a Nov 26 NATO strike on two Pakistani border posts that killed 24 soldiers, infuriating the country's powerful military which also has a tense relationship with Zardari.

(Additional reporting by Rebecca Conway in ISLAMABAD and Faisal Aziz in KARACHI, and Amena Bakr and Praveen Menon in DUBAI; Writing by Chris Allbritton; Editing by Jonathan Thatcher)