JOHANNESBURG — Tens of millions of dollars in diamond profits — perhaps more — are being secretly extracted from state-owned mines in eastern Zimbabwe, bypassing the nation’s treasury and raising fears that President Robert Mugabe is amassing wealth to help extend his 31-year reign, according to monitoring groups, diplomats, lawmakers and analysts.

Even if Mr. Mugabe’s allies in the mining ministry are telling the truth about the number of diamonds produced, the treasury was still shortchanged by at least $60 million last year, according to a budget report by the finance minister, one of the president’s chief opponents.

But the amount of money being withheld from the nation’s coffers may be much larger than that. Experts, and even some members of Mr. Mugabe’s own party, say the president’s allies are lowballing the nation’s diamond figures by millions of dollars, hoping to hide the fact that profits are being diverted for personal and political ends.

“The benefits of the diamond sales go primarily to allies of the president,” said Mike Davis, a specialist at Global Witness, a group that has extensively researched the contested mines in eastern Zimbabwe, known as the Marange fields. The strategy, Mr. Davis added, was “part of a wider attempt by people around Mugabe to seize the diamond wealth for their own political purposes, which in the short term means beating and cheating their way to another election.”