The 15-country Southern Africa Development Community had last month urged Zimbabwe to delay the July 31 elections by at least two weeks to allow adequate time to apply a raft of reforms that would ensure a free and fair vote.
But the country's top court upheld the election date that was unilaterally declared by President Robert Mugabe.
"We would have wished that our advice would have been heeded," Tanzania President Jakaya Kikwete told reporters late on Saturday after half-a-day of talks by the SADC organ on defence and security.
Putting together an election within a month "is very stressful" and to "have everything organised, you know it is quite a mammoth task", he said.
"That's why we are seeing even the incidents of the early voters, where half of them couldn't vote, partly because it is the brevity of time."
Thousands of security forces who will be working during the July 31 polling failed to cast their ballots in two days of polling early last week due to shortages of ballot papers, indelible ink and boxes.
"So it's quite going to be a tough election to organise," Kikwete said after the talks also attended by South Africa's President Zuma and Mozambique's Armando Guebuza.
But SADC, which has already deployed 360 election observers to Zimbabwe, vowed to stand by the country to ensure the vote will be "credible enough".
"We have committed to work with the people of Zimbabwe and see whatever we can do to make sure within the remaining 11 days, we can have an election that is going to be credible enough" he said. "I believe we will."
The cash-strapped country is also yet to raise all the funding needed for the election.
The much-awaited vote in Zimbabwe aims to end the uncomfortable power-sharing government between Mugabe and his archrival Prime Minister Tsvangirai, formed four years as part of a plan to end political bloodshed.
Zuma is leading the SADC mediation team on Zimbabwe, which pushed for the crunch vote.
The regional bloc had pressed Mugabe to allow time for a series of reforms that would limit the military's role in politics, strip ghost voters from the electoral roll and ensure all eligible voters were registered.
But the Saturday summit came amid a renewed attack by Mugabe of Zuma's top foreign affairs advisor Lindiwe Zulu.
Speaking at a campaign rally on Saturday, Mugabe said Zuma should rein in Zulu and that SADC should not lie about the situation in Zimbabwe.
Zulu said on Friday that there are still "challenges" in the run up to Zimbabwe's vote.
But Mugabe said "I appeal to President Zuma to stop this woman of theirs from speaking on Zimbabwe." – AFP