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"We have no reason to speculate about that," Clinton told reporters after NATO talks in Brussels.
"The information that we have is that he has sought medical treatment for a number of medical challenges and we wish him a speedy recovery.
"And certainly we expect that he will receive the treatment he is seeking and then be able to return in full health to his duties."
Zardari was expected to undergo further tests in a Dubai hospital on Thursday after suffering a minor heart attack that forced allies to deny frenzied resignation rumours.