George Bush Sr has been moved into intensive care after suffering "a series of setbacks that include a persistent fever," according to a family spokesman.
Doctors are "cautiously optimistic" about the 88-year-old former president's health, he added.
Video report by Debi Edward.
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The 100 leaders did not agree on concrete proposals, but the meeting has raised hopes for concrete international agreement next December.
The speech was OK, passionate at times and certainly well received, but did not display the passion of past speeches.
The brother-in-law of a British hostage being held by Islamic State militants in Syria, says he is scared by US airstrikes in the country.