Anthony Fauci, the head of the National Institute of Allergy and Infectious Diseases (NIAID), said officials in China had studied more than 1,000 close contacts of confirmed cases and not found any evidence of human-to-human transmission.
"That is powerful evidence because if you had a thousand contacts with someone with the flu you would be pretty sure some of them would have been infected," Fauci said in an interview with AFP.
Nevertheless, Fauci cautioned that authorities needed to be ready for the possibility of the virus mutating and spreading between humans.
"It's unpredictable as are all the influenza. One of the things we need to be concerned about is this might gain the capability of going human-to-human which up to this point has not happened and is somewhat encouraging news," Fauci said.
"But we still need to be very prepared for the eventuality of that happening."
Researchers are already developing a diagnostic test to identify H7N9, along with a vaccine, with clinical trials due in July or August.
"Work is under way on making a diagnostic test to be able to pick it up quickly," Fauci said.
"We have already started on an early development of a vaccine as we did with H5N1 years ago... Hopefully, we will never have to use it."
More than 110 people in mainland China have been confirmed to be infected with H7N9, with 23 deaths, since Beijing announced on March 31 that the virus had been found in humans.
Most of the cases have been located in eastern China, although Taiwan has reported one case. Another case has been found in southern China, while Chinese officials confirmed a further outbreak in the central province of Hunan.
Chinese authorities have identified poultry as the source of the virus and have confirmed that patients became sick from contact with infected live fowl.
A visiting team from the World Health Organization, which wrapped up a week-long visit to China on Wednesday, said there had been no human-to-human transmission but warned H7N9 was "one of the most lethal" influenza viruses ever seen.
Fauci praised Beijing for its handling of the current crisis, contrasting it to the response of the outbreak of severe acute respiratory syndrome in 2002-2003, when China stood accused of covering-up the scale of the crisis.
"It was not the case with SARS in 2003 but the transparency has been excellent," Fauci said. "I am quite satisfied with the Chinese response."
Fauci likened the current H7N9 strain of bird flu "in some respects" to the H5N1 bird flu strain of several years ago.
"The similarities are that it is fundamentally a chicken or bird flu that jumps from chicken to humans and is quite severe when it infects humans," he said.
However, Fauci added: "The difference between H7N9 and H5N1, is that H5N1 kills chickens very rapidly so it is easy to identify where the infected flocks of chickens are. H7N9 doesn't make the chicken sick, so it has been difficult to pinpoint where the infected chickens are."
There have been 566 confirmed cases of the H5N1 strain of bird flu, which killed 332 people in the world -- a mortality rate of 58 percent, compared to 20 percent for the H7N9 bird flu strain.
The H1N1 "swine flu" pandemic o 2009, which appeared in Mexico at the same time of year as the H7N9, eventually infected 60 million people throughout the world and killed more than 12,000.
The 1918 Spanish flu, which has been called one of the deadliest plagues in human history, had a mortality rate of only two percent.