JOHANNESBURG (Reuters) - The United States has terminated funding to three Zimbabwean human rights and pro-democracy groups three weeks before an election, a move that analysts say could undermine the credibility of the country’s first post-Mugabe vote.

A U.S. embassy spokesman said the decision by USAID, Washington’s aid arm, to pull the plug followed a regular internal audit that uncovered “unusual activity” and “non-compliance” in the use of funds, without providing any details.

The affected groups are the Zimbabwe Human Rights Association, Counselling Services Unit (CSU), a health clinic that provides medical treatment to victims of police torture and abuse, and Election Resource Centre (ERC).

Alongside its broader voter education work, the ERC has been working on an independent audit of the July 30 election’s register of voters, a list that has been at the center of vote-rigging allegations in the past.

The credibility of the voters roll and the election, eight months after the removal of long-time ruler Robert Mugabe in a de facto military coup, is crucial to establishing a government that is acceptable to the outside world.

Without that international seal of approval, Harare will be unable to patch up relations with the likes of the International Monetary Fund and World Bank to access the large-scale funding it needs to get its moribund economy back on its feet.

“These organizations have played a really important watchdog role,” said Piers Pigou, a Zimbabwe analyst at the International Crisis Group, a think-tank. “This has political implications.”

All three NGOs denied any wrong-doing and challenged USAID, saying it had acted unilaterally and without completing its investigations while denying the affected parties the right of reply and smearing them in the media.

“The blanket embassy statements, innuendo and allegations are deeply distressing to the board, staff and survivors of torture in Zimbabwe,” CSU said in a statement, adding that it would be opening its books to the public to prove its innocence.

The ERC said its audit of the voters’ roll had been affected although should still be concluded in time.

“We have just revised our timelines,” spokesman Tawanda Chimhini said. “We remain on track to contribute toward a constitutional, credible, free and fair election.”

U.S. embassy spokesman David McGuire declined to comment on the specific allegations or the findings of the partial investigations. He also denied that Washington was dabbling in domestic politics or guilty of undermining the vote.

“The timing obviously is unfortunate but when you’re looking at protecting your investments and you uncover wrong-doing, our philosophy is that you have to act swiftly,” he said. The funding cuts were only a “small percentage” of Washington’s annual $225 million aid package, he added.

President Emmerson Mnangagwa, who succeeded 94-year-old Mugabe after November’s military intervention, is expected to win a close vote.

(The story corrects paragraph two to make clear spokesman is from U.S. embassy, not USAID.)