John Atta Mills' death on Tuesday after falling ill led to vice president John Dramani Mahama being sworn in hours later to finish out his term, as dictated by the constitution in Africa's newest oil producer.

Mahama was said to be meeting with his cabinet on Wednesday afternoon after pledging to maintain stability in remarks after his swearing in the previous evening.

The cause of death was not given, but the 68-year-old Mills had recently traveled to the United States for what was officially a routine medical checkup and there were unconfirmed reports in local media that he had suffered from throat cancer.

His death raised a host of political questions to be resolved over the coming weeks, as Mills had been set to seek re-election in the December vote in Ghana, also a major producer of cocoa and gold.

There was widespread speculation that Mahama would now run, but the ruling party has been divided in recent months and it was not clear whether there would be a challenge for the nomination.

Analysts said a close vote in the December election was expected with much at stake.

"The trophy in this election is enormous and both sides know it...The oil is starting to flow," said Alex Vines of the London-based think-tank Chatham House.

Political questions were however temporarily put aside on Wednesday, as calls for unity and tributes to Mills, a law professor turned politician widely lauded for his integrity, filled the airwaves and newspapers.

The main opposition New Patriotic Party suspended its presidential campaign while Mahama declared a week of mourning, with flags flown at half-mast.

"I call on all Ghanaians to stand united in this moment of national loss and grief...," the opposition party's candidate, Nana Akufo-Addo, said in a statement.

Worldwide tributes also poured in, including from US President Barack Obama, who chose Ghana for his first visit to sub-Saharan Africa as president in 2009.

"President Mills tirelessly worked to improve the lives of the Ghanaian people," Obama said in a statement.

"He helped promote economic growth in Ghana in the midst of challenging global circumstances and strengthened Ghana's strong tradition of democracy."

It was not clear when the funeral would be held. The ruling party was expected to meet to plot a way forward, particularly regarding the presidential campaign.

While Mills was praised for his honesty, he had also faced criticism in recent months for what was seen as a lack of initiative.

"He was well liked, but there was also a sense of variable performance and that maybe there was a lack of energy," said Vines.

Vines said that may have been due to his health problems. Mills had recently shown signs of illness, including losing his voice and a gradual loss of weight.

The former law professor took over as Ghana's president in January 2009 after falling short in two previous campaigns.

He narrowly won the vote in 2008 with a less than one percent margin against a candidate from the party of incumbent John Kufuor, widely respected for having bowed out following his two terms in office.

In July last year, Mills was nominated to be the NDC's presidential candidate for the December vote. The primary represented the first time in the country's history that a sitting president competed for his own party's nomination.

Mills beat his only rival in the party primary, Nana Konadu Agyemang Rawlings, the wife of ex-military leader Jerry Rawlings.

The Rawlings family expressed their condolences in a statement which said they were currently attending an event in Brazzaville.

Ghana, a country of some 25 million people, joined the ranks of the world's large-scale oil producers in December 2010.

The country has begun producing oil from its offshore Jubilee field, one of the largest discoveries in west Africa in recent years. The field's operator Tullow has estimated that the field's recoverable resources amount to up to one billion barrels.