NEW YORK (CNN) -- More than 200 Afghan election officials implicated in Afghanistan's tainted presidential election will be replaced before the runoff election in less than three weeks, U.N. Secretary General Ban Ki-moon told CNN's Christiane Amanpour.

U.N. Secretary General Ban Ki-moon speaks to reporters about the run-off in Afghanistan.

"We will make sure that first of all, the Independent Election Commission will have to visit all the polling stations throughout Afghanistan," Ban said Tuesday. "And we will make sure that those polling stations from which we have received complaints will be reviewed.

"And we will try to replace, out of 380 electoral districts in Afghanistan, more than 200 district officials who have been implicated."

The U.N. leader's comments came hours after Afghanistan President Hamid Karzai agreed to a runoff with his main challenger, Abdullah Abdullah, on November 7. Karzai's decision follows a report by a U.N.-backed panel of election monitors saying there was widespread fraud in the August 20 election.

Independent analysis of that report shows Karzai's tally of votes is now below the 50 percent required to avoid a runoff.

The 200 election officials the U.N. wants to replace were recruited by Afghanistan's Independent Electoral Commission.

Ban has that the U.N. would advise the IEC not to re-recruit those officials who were implicated in fraud. The U.N. has said some flouted or ignored electoral procedures, and others were complicit in attempted fraud.

Asked by Amanpour whether it was feasible to cancel or eliminate those areas from the runoff vote where fraud was detected, Ban replied: "That would be amount to depriving the rights of vote of many Afghan people. Therefore, we cannot accept that methodology."

"We have to provide an opportunity where all the Afghan people can express of their own free will, without any intimidation, without any threat. That's the commitment of the United Nations and the international community," he said. Watch the Amanpour Webcast with Ban Ki-Moon »

The main challenger to Afghan President Hamid Karzai, Abdullah Abdullah, said Wednesday that certain conditions must be met to ensure a safe and fair runoff race, but he did not spell them out.

Abdullah Abdullah, speaking to reporters, said he would outline the conditions in coming days.

At the news conference, Abdullah repeated much of what he told CNN's Christiane Amanpour on Monday. Watch Amanpour's interview with Abdullah »

He said he was prepared to participate in the runoff election set for November 7. And he left open the door to unspecified alternatives.

If the election is not held by early November, winter would make voting impossible in some areas and force a delay until spring 2010, Afghan officials have said.

Abdullah spoke a day after Karzai bowed to Western pressure and agreed to participate in the runoff.

Western powers, notably the United States, had pushed Karzai to accept the final election results. They want to ensure that Afghanistan has a legitimate government, particularly as Washington considers beefing up its military presence there.

All About Afghanistan • Hamid Karzai