The chances of another disastrous round of voting in Afghanistan's presidential race increased dramatically today after the country's election authority defied international pressure to cut the number of polling centres in order to reduce fraud.

To the fury of UN officials in Kabul, the so-called Independent Election Commission (IEC) announced that it planned to open 155 more polling stations – up from 6,167 to 6,322 – than during the first vote on 20 August, despite repeated claims by the UN that there would be a reduction.

Ever since an official inquiry uncovered almost 1m fraudulent votes cast in favour of the president, Hamid Karzai, the country's western backers have insisted that fewer centres should be opened in the run-off next Saturday. But the IEC said assurances by Afghanistan's security chiefs that conditions had improved in some areas of the country allowed for the opening of more centres.

In the first round, polling stations operated in areas that were so insecure that election monitors were unable to deter wholesale ballot stuffing.

Karzai's main opponent, Abdullah Abdullah, had said he would only participate if 500 polling centres were closed and the head of the IEC was sacked.

UN staff said the IEC decision meant voting would take place in areas where fraud was known to have been committed, or in areas where almost no one had previously turned out to vote.

A western diplomat involved in organising the vote said the news was a "punch in the stomach, because everything we asked them to do they rejected". He said it had shattered morale among election workers, already at a low ebb after six UN members of staff involved in election preparations were killed by a Taliban hit squad in an attack on a guesthouse in Kabul yesterday. "This – on top of yesterday's incident – there really is only so much you can take," he said.

It was reported last night that the UN had ordered all non-essential staff to leave the country. Officials told the Times that about 1,000 UN workers not involved in the voting were told to return to their home countries during the election period because of the security situation.

Despite claims by Richard Holbrooke, the US special envoy to the region, that fraud would be reduced in the second round, election observers in Kabul believe a clean vote is impossible. "It's going to be a fraudulent election, we can't kid ourselves on that," one diplomat said. "The only thing that could stop it now would be if the international community as a whole decided to distance themselves from this election, and that's not going to happen."

One election observer said the US and the UK had already turned a blind eye to many of the IEC's failings in the first round of voting "for the sake of looking forward". For example, no pressure had been put on the IEC to even publish its full results for the 20 August poll.

"They will present those views to the president, collectively and individually," a Pentagon official said. "The chiefs' views in regard to [troop] resources will be an important factor."

According to an analysis by the Organisation for Security and Co-operation in Europe, the IEC's final published tables has missing or incomplete results for more than 4,000 polling stations. Nor has the IEC explained why it did not follow orders from the UN-appointed watchdog, the Electoral Complaints Commission, to annul results from 210 polling stations.

The IEC only cancelled votes cast in 147 stations, which probably ensured that Karzai's final official result for the first round was only slightly lower than the 50% overall majority he desperately wanted, rather than the result privately calculated by the ECC of 48.3%.

Meanwhile, US military chiefs plan to present recommendations on troop strength and strategy in Afghanistan to President Barack Obama today, officials said, a sign that the White House's deliberations may be nearing a conclusion.

The US defence secretary, Robert Gates, and the chairman and vice-chairman of the joint chiefs of staff, Admiral Mike Mullen and General James Cartwright, will be joined at the White House meeting by the top leaders of the army, marine corps, navy and air force.