Pakistani President Asif Ali Zardari was undergoing what his office said on Wednesday were routine medical tests in a Dubai hospital, but which fuelled rumours of his possible resignation.

Zardari's office said a news web report, which kicked off 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.

Twitter and other social media were awash in contradictory information. In one span of 20 minutes, micro-bloggers had placed Zardari in Dubai, Abu Dhabi and Saudi Arabia.

Some speculated the president would travel to the Britain, or return to Pakistan by the end of the week. Others suggested the president had suffered a minor heart attack and may resign.

Rumours also circulated on Twitter of an army takeover on Tuesday night, but that was quickly recanted by the original poster.

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 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.

Tensions between Pakistan's civilian government and military have bedevilled the nuclear-armed South Asian country for almost its entire 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 and Faisal Aziz; Writing by Chris Allbritton; Editing by Nick Macfie)