Wells Fargo & Co.'s surprisingly strong first quarter has not quieted worries that its reserves are too thin to absorb rising loan losses.

Even at $4.6 billion — or $1.3 billion higher than quarterly chargeoffs — the $1.3 trillion-asset San Francisco company's first-quarter provision may not be enough to cover credit costs, particularly losses on option adjustable-rate mortgages and other problem assets it inherited when it bought Wachovia Corp. last year, according to a team of Friedman, Billings, Ramsey & Co. Inc. analysts led by Paul Miller. In a note to clients issued last week, the analysts wrote that the provision should have been $6.25 billion.

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