Our correspondent said late night meetings had been held at the presidential palace in Kabul in the wake of the ECC report to discuss the president's response.
"I understand that key ambassadors are now meeting with the UN to discuss what is the next step, what is the way forward," he said.
Karzai, who had earlier proclaimed victory in the poll, is expected to give his reaction to the ECC's findings early on Tuesday.
The head of the ECC has said the results are binding, but it remains to be seen whether the Afghan-led Independent Election Commission (IEC) will accept the findings and order another poll.
Fazel Sancharaki, Abdullah's campaign spokesman, welcomed the ECC's findings, saying: "This is a step forward."
But Waheed Omar, a spokesman for Karzai's campaign, dismissed claims that the ECC report meant a runoff would be necessary.
|The ECC report has called for thousands of Karzai votes to be scrapped [AFP]
"I do not know how any diplomatic source can come out with a result out of what has been said today," he told Al Jazeera.
"We will keep our reaction reserved for when the final, certified results of the elections are announced [by the IEC] and that will be legally binding for us."
The IEC, the Afghan electoral body which organised the vote, is still to announce the official results, but the ECC's findings are seen as key to the outcome.
Hillary Clinton, the US secretary of state, said she expected an announcement from Karzai on Tuesday would lead to a quick solution to the election deadlock.
"I am going to let him do that but I am encouraged at the direction that the situation is moving," Clinton told reporters.
"I am very hopeful that we will see a resolution in line with the constitutional order in the next several days."
Karzai's camp has already criticised the ECC and some fear that the Karzai-influenced election commission may refuse to call a runoff, further delaying formation of a government.
Speaking to Al Jazeera, Karzai spokesman Waheed Omar said that he was not aware of any dispute between the ECC and the IEC.
Soon after the ECC's findings were released, Britain hailed the report, saying the IEC should quickly "implement those orders" to invalidate thousands of ballots.
The US has said it is "incredibly important" for the world to see a legitimate Afghan government as soon as possible.
"It is now up to the Afghans to demonstrate they believe in that legitimacy as well," Robert Gibbs, the White House spokesman, said.
PJ Crowley, the US assistant secretary of state, told Al Jazeera: "We believe that the Afghan government should accept the results when they are released and should proceed with the process wherever those results lead.
"If Karzai and his main competitor, Abdullah Abdullah, can reach an agreement on a unity government, what we want to see ultimately is a legitimate government and a more effective government in Afghanistan that is willing and able to deliver services in support of its people.
|The US has said resolving the deadlock is key to future troops deployments [GALLO/GETTY]
US election monitoring group Democracy International, which had some 60 observers in Afghanistan monitoring the ballot, also heralded the ECC's findings and called for a runoff vote as soon as possible.
"Democracy International ... believes the ECC audit decisions should result in a runoff election, according to Afghanistan's electoral law," the group said in a statement.
Glen Cowan, co-founder of democracy international, told Al Jazeera that election papers for a runoff were already in Kabul and a vote could be organised as early as November 5.
Preliminary results last month had showed Karzai winning the election with more than 54 per cent of the vote to Abdullah's 28 per cent, but allegations of massive fraud prompted the ECC investigation.
Many Afghans have expressed frustration with the almost two-month delay to the announcement of the country's election results.
'Credible and legitimate'
Hundreds of Karzai supporters protested in the south over the weekend, calling for the IEC to release the results quickly and saying they will reject a second round.
They gathered in the main street of the southeastern city of Spin Boldak on Sunday, shouting: "We want the result!" and "Karzai is our leader!"
The election deadlock has also complicated a major US review of its policy in Afghanistan, where it is fighting the Taliban.
The US is considering a request by General Stanley McChrystal, the top US and Nato ground commander, to commit an extra 40,000 troops in order to step up its operations there.
But Rahm Emanuel, the chief of staff to the US president, has suggested that the US may not commit more troops to Afghanistan until a "credible and legitimate" government is in place.
Speaking on CBS talk show Face The Nation on Sunday, Emanuel said the overriding question was not "how many troops you send, but do you have a credible Afghan partner".