On the fourth day of his triumphant return to his father's homeland, the rising political star toured the offices of a local media group that was raided earlier this year and planted a tree with environmental champion Maathai.

Speaking at Nairobi's Standard Media Group, Obama offered a pep talk to employees concerned by growing assaults on press freedom, particularly in Sudan where a journalist from his home state of Illinois was arrested this month.

"Journalists undergo various challenges on their day-to-day basis," said the only African-American in the US Senate and potential Democratic Party presidential nominee.

"As you are aware, one of our reporters from the Chicago Tribune is being detained in the Sudan allegedly on charges of espionage," Obama said, calling for a quick resolution of the case of Paul Salopek.

Salopek, a correspondent for the Chicago Tribune on assignment for National Geographic in Sudan's war-torn Darfur region, was charged at the weekend with espionage, reporting false information and entering Sudan without a visa.

"This is a matter that the State Department and journalists' organisations are taking seriously," Obama said, before returning to the case of the Standard, which was raided in March after reporting on government corruption.

"This is a problem that is worldwide, it is not isolated to Kenya," he told journalists, calling the raid "unfortunate." "We should press governments across the world to be more accountable and transparent."

"The media should not be intimidated by these challenges because they are things they face quite often," Obama said, urging the press to "uphold ethics governing them because they are answerable to the people and not governments."

"Press freedom is like tending a garden, it continually has to be nurtured and cultivated," he said. "The citizenry has to value it because it's one of those things that can slip away if we're not vigilant."

Later, after planting a tree with Maathai, Kenya's former deputy environment minister who won 2004 Nobel Peace Prize for her work in planting trees to promote development, Obama lamented ecological losses around the world.

He singled out US President George W. Bush's refusal to join the UN Framework Convention on Climate Change (UNFCCC) and its offshoot, the Kyoto Protocol, which sets out measures for tackling greenhouse gas pollution.

"My attitude is that even if the Kyoto Protocol wasn't the perfect arrangement, we should have continued to negotiate," he said after the tree-planting with Maathai at Nairobi's vast Uhuru Park.

He urged developed countries to invest in eco-friendly technologies that could at once help protect the environment and create jobs while meeting the challenge of addressing global warming.

Obama, 45, is due to give a "major speech" on US-Africa relations later Monday at the University of Nairobi, where students, like others in Kenya were expected to give their new "favorite son" a rousing welcome.

The junior senator, son of a Kenyan goat herder-turned-economist, has been given a rock star reception here where thousands have thronged his public events, cheering him with hero-like reverance.

He has already addressed key issues such as government corruption, poverty and the fight against HIV/AIDS with government officials, slum dwellers in the capital, his father's western home region, and Kenya's drought-stricken north.

He wraps up his five-day trip to Kenya with a visit to the east African nation's famed wildlife-rich Maasai Mara National Reserve on Tuesday before heading on to Chad and Djibouti.

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