The US space agency Thursday vowed to reconsider the applications of Chinese scientists who were denied access to a NASA conference on security grounds, sparking a boycott by leading American astronomers.

NASA administrator Charles Bolden said the situation was "unfortunate" and pledged to take a fresh look at the applications once the US government reopens.

The government has been partially shut down since October 1 amid a US political crisis, sending home 97 percent of NASA staff without pay as well as hundreds of thousands of federal employees.

"It is unfortunate that potential Chinese participants were refused attendance at the upcoming Kepler Conference at the Ames Research Park," Bolden wrote in an email to Congressman Frank Wolf, seen by AFP.

"Mid-level managers at Ames, in performing the due diligence they believed appropriate following a period of significant concern and scrutiny from Congress about our foreign access to NASA facilities, meetings and websites, acted without consulting NASA HQ (headquarters)," he continued.

"Upon learning of this exclusion, I directed that we review the requests for attendance from scientists of Chinese origin and determine if we can recontact them immediately upon the reopening of the government to allow them to reapply."

Bolden said any scientists who meet "the clearance requirements in place for foreign citizens will be accepted for participation."

The conference is to be held November 4-8 at a NASA facility in northern California.

The applications of six Chinese scientists were denied due to what organizers said was a March 2013 order for a moratorium to visits to NASA facilities by citizens of several nations including China.

The basis for the ban was called into question on Tuesday by Wolf, who authored related legislation in 2011 that he said restricted space cooperation with the Chinese government and Chinese companies but not individuals.

The moratorium and other additional security measures were issued earlier this year by Bolden following a potential security breach at a NASA facility in Virginia by a Chinese citizen, and should have been lifted by now, Wolf said earlier this week.

Some leading US astronomers have vowed to boycott the conference next month on the basis of the denied applications.

One of them, Debra Fischer of Yale University, told AFP that one of her post-doctoral students was among those whose application was denied.

Beijing's foreign ministry described the application denials as discriminatory, and said academic meetings should remain free of politics.