It is an image famous in a thousand postcards: giraffe, rhino and zebra pacing the savannah with city skyscrapers towering in the background.

But flanked by one of the continents fastest growing cities, Kenya's capital Nairobi, east Africa's oldest national park is under threat.

"There is huge pressure on the park," said conservation activist Paula Kahumbu, who heads the Wildlife Direct campaign group.

Set up by British colonial settlers in the 1940s, pressure now comes from all sides: roads, railways, factories and houses.

The park, some 117 square kilometres (45 square miles), is a wilderness where buffalo and rhino roam just seven kilometres (four miles) from the bustling high-rise city centre.

But like countries across the continent, Kenya is weighing the difficult balance between conservation and development.

The century-old colonial railway yard is now a traffic-clogged major city growing at breakneck speeds.

To the south, the reserve has already had to face the development of a large urban area, pressing on a key wildlife corridor for animals moving to find grazing.

Now fresh infrastructure projects threaten the park: a major road bypass and expansion of a railway line, seen as vital to modernise freight lines bringing goods to Kenya and onwards to landlocked east Africa nations.

- Shady land deals -

"The more we reduce the park, the more the animals' territory shrinks," said Ali Tanvir, president of the Friends of Nairobi National Park group.

Supporters say the projects are crucial to decongest the capital of four million people, but the road and rail could slice through the reserve.

"Kenya is a developing country, we need roads, railway lines, bridges," said MP Francis Nyenze.

"But it is unfortunate that most of the major infrastructure projects in Kenya will swallow parts of the park."

Nairobi prides itself on hosting the regional headquarters of multinational companies, and of being the powerhouse driving the economy east Africa.

But it is crippled by traffic jams, with vehicles coming from the Indian Ocean port of Mombasa to the rest of Kenya -- as well as to landlocked Uganda, Rwanda, South Sudan -- all travelling through Kenya's capital.

Conservationists do not oppose the need for new transport links, but question how those plans are being implemented.

Shadowy deals have seen land sold and houses sprout up at times apparently unchecked. Land in Kenya is both increasingly expensive and a highly political issue.

"How have people been able to acquire titles to the land?" Nyenze asked.

Any development encroaching on nationally protected reserve must be approved by parliament, said Nigel Hunter, from the East African Wildlife Society.

Although fenced in on the city side, the park is open-sided elsewhere else to allow the annual wildlife migration in search of grazing.

- 'Disappear like dinosaurs' -

"We want the rules to be respected," Hunter said, adding that if granted, land should be opened elsewhere to allow animals still to move.

Without that, the parks risks becoming an "island" as if a giant zoo, he said.

Home to more than 550 species of birds and attracting some 150,000 visitors a year, activists say the park also acts as the lungs of the city, bringing in fresh air to an increasingly crowded capital.

But the challenges and threats are huge.

New homes and fences block ancient wildebeest migration routes, and so many cheetahs have been killed on the increasingly busy roads there are none left in the park, said Kahumbu.

"Roads are fragmenting the ecosystem and preventing animals from moving," she said.

Conflicts between lions and livestock communities living close to the park have also grown. And if the steady encroachment on the park was not enough, animals inside are directly targeted.

Amid a wave of rhino and elephant killings across the country, armed poachers have also slaughtered rhino in the heart of the park, despite it being close to the headquarter of the Kenya's wildlife rangers.

For MP Nyenze, the future is gloomy, fearing the park could disappear within 20 years.

"If the voices are not many and loud enough, the destruction will go on and elephants, lions and so forth will disappear like dinosaurs," he said.

"It will be a world without wildlife and we will lose all this biodiversity once and for all."