Al-Qaeda Islamist militants have renewed their threat to bomb targets in Nigeria, Africa's top oil producer, a newspaper reported quoting the national police chief.

The United States embassy in Nigeria said last September the country was at risk of "terrorist attack" and Osama bin Laden once named the world's eighth biggest oil exporter as ripe for jihad or Islamic holy war.

"The al-Qaeda network has threatened to send time bombs to Nigeria ... CPs (commissioners of police) of all the commands should be on the alert and ensure that these items (bombs) do not pass through their end," the Punch newspaper quoted Inspector General of Police Mike Okiro as saying.

He gave no details of what the targets might be, but he told a group of senior officers that intelligence reports showed the threat was real.

A number of suspected jihadists have been arrested by police and the State Security Services (SSS) in recent years, but the cases have dragged on in the courts and there have been no convictions. No conclusive evidence of al-Qaeda's presence in Nigeria has been made public.

Five Islamist militants with suspected links to al-Qaeda are on trial in the capital Abuja for plotting attacks on government targets in Africa's most populous country.

The men were arrested in November by the SSS in mainly Muslim northern Nigeria. Three of them have also been charged with training in Algeria with the Salafist Group for Preaching and Combat (GSPC) between 2005 and August 2007.

The GSPC renamed itself al-Qaeda in the Islamic Maghreb in January 2007.

The charge sheet said the five militants, all in their early 30s, "did conspire to commit terrorist acts," and said three of them trained in Algeria "with intent to attack government facilities and cause insurrection in Nigeria".

Another charge said the militants had an AK 47 rifle, ammunition, dynamite, fertiliser "and 11 explosive devices" which they planned to use to attack government facilities and installations in the southern cities of Lagos and Ibadan.

Nigeria's 140 million population is roughly equally split between Christians and Muslims. The two groups usually live side by side peacefully, but there are occasional outbreaks of sectarian conflicts.

Tensions heightened in 2000 after 12 mainly Islamic northern states began a stricter enforcement of sharia, alienating sizeable Christian minorities. Thousands were killed in sporadic riots across the country.

CLEANER air due to reduced coal burning could help destroy the Amazon this century, according to a study in the journal Nature.

The research identifies a link between reduced sulfur dioxide emissions from coal burning and increased sea surface temperatures in the tropical North Atlantic that boosts the drought risk in the Amazon rainforest.

With the rainforest already threatened by development, higher global temperatures could tip the balance, it says.

"Generally, pollution is a bad thing, but in this case improving the air may have ironically led to a drying of the Amazon," said Peter Cox, a researcher at the University of Exeter in Britain, who led the study.

“It shows you have to deal with greenhouse gases.”

The Amazon - the world's largest tropical rainforest - plays a critical role in the global climate system because it contains about one 10th of the total carbon stored in land ecosystems.

The researchers used a climate-carbon model to simulate the impacts of future climate change on the Amazon and compared it to data from a 2005 drought that devastated a large chunk of the rainforest.

They estimated that by 2025 a drought on the same scale could happen every other year and by 2060 such a crisis could hit nine out of every 10 years - enough to turn the rainforest into savannah grassland, Cox said.

In the pre-industrial age, the Amazon was less vulnerable. But higher temperatures and destruction of the forest make droughts far more likely than in the past, the researchers said.

“The Amazon is said to be the lungs of the planet,” Cox said. “You don't want to damage it.”

The researchers believe that efforts to clean up sulphate aerosol particles from coal burning at power stations in the 1970s and 1980s helps to explain the threat.

The pollution predominately in the northern hemisphere had limited warming in the tropical north Atlantic, keeping the Amazon wetter than it normally would have been.

But with that protection evaporating due to cleaner air and as greenhouse gases fuel global warming, the rainforest now faces a deadly drought risk, the researchers said.

“Reduced sulphur emissions in North America and Europe will see tropical rain bands move northwards as the north Atlantic warms, resulting in a sharp increase in the risk of Amazonian drought,” Chris Huntingford, a researcher at Britain's Centre for Hydrology and Ecology said.

The findings highlight the need to deal not only with greenhouse gas emissions but also with the direct destruction of the rainforests as well, the researchers said.

They said 20 per cent of global carbon dioxide emissions stem from burning of trees to build new homes and roads as development pushes farther into the delicate region, they added.

“You can argue there is a greater urgency to deal with the deforestation issue in our model,” he said.