The United States warned its citizens Monday not to travel to Olympic host Japan, citing the growing risk of the COVID-19 pandemic in the Asian nation just two months before the Games begin.

The warning came in a travel advisory issued by the State Department as Japan, which has been criticized for its slow inoculation rate, opened its first mass vaccination centers in a push ahead of the Olympics, which were postponed last year due to the pandemic.

The decision was based primarily on government health advice, as well as "secondary factors such as  commercial flight availability,  restrictions on US citizen entry, and impediments to obtaining Covid test results within three calendar days," the advisory said. 

Just two percent of Japan's population of 125 million has been fully vaccinated so far.

It began giving the Pfizer shot in February first to medical workers and then over-65s, whom the government aims to finish inoculating by late July, when the Olympics begin.

But ministers say the Games do not figure in their rollout schedule, and no date has been announced for other age groups.

Japan has seen a relatively small coronavirus outbreak, with around 12,000 deaths overall, but a recent surge in infections has put hospitals under strain.

Tokyo, Osaka and eight other regions are under a state of emergency curbing commercial activity until the end of May, with reports saying the measures could be extended for another three weeks.

Public opinion is largely opposed to holding the Olympics this summer but organizers say the event can be held safely.

The majority of athletes and others staying in the Olympic village will be vaccinated before they enter Japan, but inoculation is not required to participate.

A State Department spokesman said Washington "respected the decision to delay the Games last summer and we understand the careful considerations that the Japanese government and the International Olympic Committee are weighing as they prepare for the Tokyo Olympics this summer."

"President Biden proudly supports the US athletes who have trained for these Games and will be competing in the best traditions of the Olympic spirit," he said.