Ranil Wickremesinghe said the government was discussing a replacement for a 1995 Status of Forces (SOFA) arrangement that would allow the militaries of the two countries to access each other's ports and airports with fewer formalities.

The prime minister said negotiations were ongoing as the new document contained provisions Sri Lanka was "not able to accept", without elaborating on what they were.

His comments came days after President Maithripala Sirisena said he would not let the government headed by his pro-West arch-rival conclude any deal giving US forces freer access to Sri Lanka.

"I will not allow the SOFA that seeks to betray the nation. Some foreign forces want to make Sri Lanka one of their bases. I will not allow them to come into the country and challenge our sovereignty," Sirisena, who faces an election this year, said at a rally last weekend.

The United States last year granted $39 million for maritime security to Sri Lanka as China develops its strategic hold in the region.

US concern has increased as China steps up investment in ports and other projects on the island, which is a key link in Beijing's ambitious "Belt and Road" infrastructure initiative.

China has vowed to keep providing financial help, including loans, to Sri Lanka despite warnings about the nation's mounting debt.

Sri Lanka in 2017 gave a 99-year lease on a strategic port to Beijing because it could not repay Chinese loans for the $1.4 billion project.

Hambantota port straddles the world's busiest east-west shipping route and also gives a strategic foothold to China in a region long dominated by India.