The US track and field team has scrapped plans for pre-Olympic training in Japan over concerns about safety during the pandemic, Japanese officials said.

US athletes were supposed to train in Chiba, outside Tokyo, before the pandemic-postponed Games open on July 23.

But the Chiba regional authority said the team canceled "because of concerns over athlete safety as the coronavirus pandemic continues around the world with no prospects of winding down."

"Although the cancelation is extremely regrettable, we think it is the best decision... given the current situation," it said.

The American team (USATF) said its plans had been badly disrupted over the past year, adding that athletes had been encouraged to train in the US.

"With the uncertainty surrounding competitions in 2020 and 2021, USATF provided domestic competitive opportunities... and encouraged Team USATF athletes to stay in the US and train," a spokesperson said in an email.

Pre-training camps across Japan have been canceled either by prospective host towns or the athletes involved over virus concerns as the country battles a fourth wave of infections. 

Tokyo and several other parts of Japan are under a virus emergency, and the government is facing pressure over its slow vaccine rollout.

Organizers say anti-virus measures will ensure the Games are held safely and point to a string of recent test events, including some with international participants which were held without producing virus clusters.

"We are now very much in an implementation phase," International Olympic Committee spokesperson Mark Adams told reporters in a briefing.

"With 78 days to go, we are fully concentrated on the last implementation phase."

The Japanese public, however, remains opposed to holding the Games this summer, with polls showing most in favor of cancelation or a further delay.