Colombo (AFP) - President Xi Jinping was Tuesday due in Sri Lanka where he will launch construction of a Chinese-backed $1.4 billion port city as he promotes his vision of a "maritime silk road" in the face of growing competition from Japan and India.

China is increasingly asserting its influence in the Indian Ocean, with Sri Lanka a midway point on one of the world's busiest international shipping lanes.

It has overtaken India to become the largest investor in Sri Lanka, and has also consistently supported Colombo in its efforts to resist a UN investigation into alleged war crimes. India, by contrast, has pushed Sri Lanka to ensure accountability.

Xi, the first Chinese head of state to visit Sri Lanka in 28 years, said no country should be allowed to intervene in the affairs of the small island nation, which has historically come under the influence of regional power India.

"China... resolutely opposes any move by any country to interfere in Sri Lanka's internal affairs under any excuse," Xi said in a letter published on the front-page of Sri Lanka's state-run Daily News.

"China and Sri Lanka need to turn our wishes into a driving force and enhance exchanges and cooperation between us in maritime, business, infrastructure, defence, tourism and other areas to accelerate the renewal of the maritime silk road for the benefit of our two countries and peoples."

Xi's call came amid repeated Sri Lankan assurances that relations were based on commercial rather than security considerations.

The Chinese president arrives from the neighbouring Maldives, where he secured support for his "21st century maritime silk road", an initiative that seeks to secure trade routes, largely through economic diplomacy.

During his visit, Xi will launch construction of a new $1.4 billion port city being built with funding from Beijing.

The new development will go up next to a $500 million Chinese-owned container terminal, the only mega port in South Asia.

But China faces competition for influence in the region from both India and Japan, which relies on the same maritime trade routes.

Japanese Prime Minister Shinzo Abe travelled to Sri Lanka earlier this month, securing an agreement that the two countries would forge stronger maritime links.

India's new Prime Minister Narendra Modi, meanwhile, has embarked on a regional charm offensive since taking office in May to restore relationships critics say the previous government neglected.

- 'Nothing to fear' -

Xi said he was keen to experience the charm of Sri Lanka, calling it the "splendid pearl of the Indian Ocean."

China has been accused of developing ports around India in a "string of pearls" circle of influence to counter New Delhi.

But Sri Lanka's economic development minister Basil Rajapakse said ahead of the visit his country's close ties with China should not be a cause for concern in New Delhi.

"We are looking for trade, investment and tourism," Rajapakse told AFP. "India is also looking to China so I do not see anything wrong in our having close ties with them."

The ambitious port city project being developed by Sri Lanka Ports Authority (SLPA) with Chinese backing will include a luxury marina and a Formula One race track.

SLPA chief Priyath Bandu Wickrama said the port would be "based on commercial interests".

"We will not allow our ports for any military purposes," he told AFP. "There is nothing for anyone to fear."

Beijing has already financed the construction of Sri Lanka's second international airport and another deep sea port in the southern district of Hambantota, the home constituency of President Mahinda Rajapakse.

During his visit, Xi is expected to commission the third phase of a 900-megawatt coal power plant China has built in the island's north-western cost.

But a military aircraft maintenance facility Sri Lanka sought to establish with Chinese help has been in limbo since India privately objected to its chosen venue, the eastern port district of Trincomalee.

Trincomalee is a natural deep sea harbour which allied forces used as a staging post during World war II and is still considered strategically important.