Sri Lanka's opposition Tuesday accused China of destroying the island's pristine beaches and warned Beijing its $1.4 billion port investment could be in jeopardy if they returned to power.

Former prime minister and opposition leader Ranil Wickremesinghe said his party would take a fresh look at "haphazard reclamation" of land near Colombo harbour for a new port city.

"If you build the port city... the (western) coast (of Sri Lanka) will be completely destroyed. Please remember that," Wickremesinghe said. "We have to reconsider this."

Construction of the port city is underway.

He accused the project of going ahead without any regard for environmental damage it causes to the coastline where hundreds of luxury tourist hotels are located.

Chinese President Xi Jinping during a brief visit to Sri Lanka in September launched construction of the $1.4 billion port city that will give Beijing a firmer foothold in the Indian Ocean region.

Wickremesinghe made it clear that the opposition would "take a fresh look at the development project" if they were returned to power in the January 8 presidential election.

President Mahinda Rajapakse, who relies heavily on Chinese funding for major infrastructure work on his Indian Ocean island, is seeking an unprecedented third term, but faces an unexpected challenge from his former health minister Maithripala Sirisena.

Sirisena has the backing of a combined opposition led by Wickremesinghe who wants to be prime minister under a new government.

Colombo Port City, Sri Lanka's largest single foreign investment, is being built alongside an already existing Chinese-built container terminal, the only mega port in South Asia.

As part of the deal with Sri Lanka, China will gain ownership of one third of the total 233 hectares (583 acres) of reclaimed land that the new port city will occupy.

Sri Lanka is a midway point on one of the world's busiest international shipping lanes that Beijing wants to secure as a maritime silk road for the 21st century.

Colombo hopes the city, which will have a Formula One track and a luxury marina, will attract another $5 billion in foreign investment for property development.

China's huge investments in Sri Lanka and other South Asian nations, in India's neighbourhood, have caused unease in New Delhi.

Some officials in New Delhi have voiced fears in the past that China's growing engagement in the region is a deliberate strategy to encircle India.