Japan is considering an additional payment of up to $700 million to the Covax Facility, a U.N.-backed program intended to ensure a fair supply of coronavirus vaccines in developing countries, government sources said Wednesday.

Prime Minister Yoshihide Suga will unveil the plan in an online meeting of global leaders on vaccine rollouts slated for June 2, the sources said.

The envisaged payment will come on top of Japan's initial offering of $200 million to Covax, or the COVID-19 Vaccine Global Access, Facility.

Japan cohosts the June 2 summit with the Gavi vaccine alliance of governments, international organizations, companies and charities.

High on the agenda will be the planned delivery of 1.8 billion doses of vaccine to cover 30 percent of the population of developing countries this year, according to the Japanese Foreign Ministry.

While members have so far secured a total of $6.6 billion to that end, the leaders will discuss ways to pool the remaining sum of $1.7 billion required, it said.