"I'm not sure if it's a stomach virus yet or something like that," Dan Bartlett, counselor to the president, told reporters. "He's just not feeling well in his stomach."
Although ill, the president did meet for an hour with French President Nicolas Sarkozy but, as a precaution, the meeting took place in Bush's private room.
"President Bush was slightly unwell this morning and will return to work as soon as he can," Sarkozy told reporters.
Bush's doctor, Dr. Richard Tubb, is monitoring the president's health, Bartlett said. He said Bush hoped to rejoin the other leaders at the summit.
Bartlett joked that Bush was staying in his room because he did not want to follow in the footsteps of his father, former U.S. President George H.W. Bush.
Bush's father became ill and collapsed in the international spotlight in January 1992 during a summit in Tokyo. The president called his fainting, which was captured on film and run repeatedly on television, "a little tiny bout of the flu" adding, "That's all there is to it."
A videotape of the elder Bush's collapse at a state dinner showed him toppling unconscious from his chair and vomiting as Barbara Bush rushed to aid him.
Bartlett said the current president's illness is "not serious." He was already dressed this morning when he began feeling ill, but taped his radio address and at this point has no changes to his travel plans for the day.
The Bush-Sarkozy meeting was their first since Sarkozy's election less than a month ago.
Bush "felt that they established a real personal rapport," Bartlett said, adding that they discussed a myriad of issues, including Iran, Syria, Lebanon, Iraq, Afghanistan, Darfur, trade and missile defense.
Bartlett said Sarkozy was interested in hearing about Putin's missile defense proposal, but didn't go beyond that. Bush, who is scheduled to head to Poland later Friday, will discuss the matter with the Polish president, he said.
"It'll be an important consultation but don't expect to have definitive answers to a very complicated set of issues," Bartlett said. "This is going to be a continuing dialogue with all interested parties. The president's meeting with the Polish government is very timely."
The two also discussed the future of the province of Kosovo, where the ethnic Albanian majority is seeking independence from Serbia, Sarkozy said.