It was unclear, however, whether the Afghan-led Independent Election Commission would accept the findings of the fraud panel and announce a runoff. Karzai's spokesman said it was too soon to make a judgment based on the figures released by the panel.
That could mean a further delay in forming a new government that the U.S. believes is needed to help combat the growing Taliban insurgency. A protracted crisis could also lead to political unrest.
"It's going to be incredibly important for the world to see that Afghan leaders are willing to make this process legitimate," White House press secretary Robert Gibbs said in Washington on Monday.
The White House has also said no decision on sending more U.S. troops to Afghanistan would be made before the election crisis is resolved - a stance reiterated Monday by the civilian chief of the NATO military alliance.
U.S. Sen. John Kerry, chairman of the Senate Foreign Relations Committee, who was among a host of international envoys in Kabul on the weekend urging the president to accept the fraud rulings, returned Monday to resume meetings with Karzai, the U.S. Embassy said.
Two international officials familiar with the investigation by the U.N.-backed Electoral Complaints Commission told The Associated Press that the findings showed Karzai falling below the 50 percent required to avoid a runoff with his chief rival, former Foreign Minister Abdullah Abdullah.
An independent calculation by an election monitoring group, Democracy International, showed Karzai with 48.3 percent, or about 2.1 million votes, after more than 995,000 of his votes were thrown out for fraud.
Overall, about 1.3 million votes of the more than 5 million ballots cast were voided. Abdullah lost more than 201,000 votes, but his percentage rose to 31.5 percent from 27.8 percent previously.
Preliminary results released last month showed Karzai winning the Aug. 20 election with more than 54 percent. However, allegations of voter coercion and ballot box-stuffing prompted the fraud investigation and held up a final proclamation of a winner.
Abdullah campaign spokesman Fazel Sancharaki welcomed the fraud panel's findings and said they showed Karzai's percentage of the vote was 48 percent.
"This is a step forward, now it is up to the IEC to announce the final results," Sancharaki told The Associated Press. He said it would be illegal for the IEC to reject the fraud panel's findings.
Karzai campaign spokesman Waheed Omar said, "I don't think we can make any judgment based on the figures announced today."
Investigators only released raw data from their findings, but it was clear that hundreds of thousands of Karzai votes were voided.
Afghan law declares the U.N.-backed panel the final arbiter on fraud allegations. However, Karzai supporters on the election commission have argued that the partial recount is beyond the normal complaint process and that they must have a say in whether the findings are accepted.
Grant Kippen, chairman of the Electoral Complaints Commission, said he did not see any legal way for the IEC to reject the results.
"Our decisions, our orders, are final and binding according to the law," Kippen said. "We've followed the law very clearly, very precisely. My sense is that the IEC is going to follow the law as well."
He declined to talk about whether the data showed the need for a runoff, saying it was up the IEC to make that call.
Hundreds of Karzai supporters protested in the president's native southern province over the weekend, calling for the electoral commission to release 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!"
Ali Shah Khan, a tribal leader from the area, said the protesters believed the August vote was fair and that foreigners were delaying the results to unseat Karzai.
"We know they don't want President Karzai because he is a strong leader and he is working only for the people of Afghanistan," Khan said. "The foreign countries want a weak leader for Afghanistan. After that they can do whatever they want."
The White House says President Barack Obama will not send more U.S. troops until a credible government is in place.
"There is a need for rapid decisions (but) it's important to stress that there is a strong need for the international community to have a credible and accountable government in Kabul to deal with," NATO Secretary-General Anders Fogh Rasmussen said in Brussels.
French Foreign Minister Bernard Kouchner and Kerry were in Kabul on the weekend urging the Afghans to resolve the standoff quickly.
Sancharaki, the Abdullah spokesman, said the U.N.-backed panel "is under threat" from Karzai. He alleged the president was urging the election commission not to accept the results of the fraud probe.
"He's telling them not to accept the findings if they show less than 50 percent for him. That's why the IEC is not accepting the final report," he said. "There is no end to this misery. Negotiations are still going on, but there is no agreement."
Omar, Karzai's campaign spokesman, has denied any presidential interference.
Afghan officials have said they could organize a runoff in about two weeks, which is close to the start of winter. When snows fall in the high mountain passes, it will become all but impossible to organize and conduct an election until the spring. A second-round vote would also run the risk of Taliban attacks on voters such as those carried out during the first ballot.
For those reasons, Western diplomats have urged the two sides to reach a power-sharing agreement which would avoid a new vote and bring an end to the crisis. Both sides have expressed a willingness to talk but are said to be far apart.
Karzai has said he would be willing to offer posts to the opposition in a new government - which falls short of a real coalition with clearly defined powers.
As the debate rages, deadly fighting continues.
On Monday, Taliban militants set fire to 15 trucks carrying supplies to a military base in eastern Ghazni province, according to local official Sahib Khan. Afghan security guards killed two militants during the fighting.
Two Afghan security troopers were killed in a gunbattle overnight with Taliban fighters near Ghazni city, provincial spokesman Ismail Jahangir said.