Top government officials will meet on Saturday to discuss Abdul Rahman, 41, who was denounced by his own parents and arrested under Islamic Sharia law on his return to Afghanistan from Germany two weeks ago.
The official said: "He is likely to be released soon."
Abdul Rahman, a former Muslim, reportedly became a Christian 16 years ago in Germany. He faced execution under Sharia law, on which the Afghan constitution is partly based, if he failed to revert.
The case has attracted widespread international condemnation, especially from the US which led the campaign to remove Afghanistan's Taliban government.
American Christian lobby groups are urging George Bush, the US president, to do more to save Abdul Rahman, a former medical aid worker, whose trial will begin next week.
But Karzai cannot ignore conservative proponents of Islamic law or appear to bow too readily to outside pressure.
Religious and political figures meeting at a Kabul hotel, including former prime minister Ahmad Shah Ahmadzai and Shia cleric Asif Mohsenia who commanded anti-Soviet forces in the 1980s, have said the government should ensure that Islamic law is enforced.
It said if its demands were ignored, "the Muslim people of Afghanistan would consider struggle their legal and religious duty".
Karzai is under Western pressure
to save Abdul Rahman's life
Death is the punishment stipulated by Sharia, or Islamic law, for apostasy. The Afghan legal system is based on a mix of civil and sharia law.
Virtually everyone interviewed in a small sample of opinion in several parts of the deeply conservative, Muslim country on Friday said Abdul Rahman should be punished.
Several clerics raised the issue during weekly sermons in Kabul on Friday, and there was little sympathy for Abdul Rahman.
"We respect all religions, but we don't go into the British embassy or the American embassy to see what religion they are following," said cleric Enayatullah Baligh at Kabul's main mosque.
"We won't let anyone interfere with our religion, and he should be punished."
Freedom of religion
Analysts say they doubt the man will be executed and his case could hinge on interpretations of the new constitution, which says "no law can be contrary to the sacred religion of Islam".
It also says Afghanistan will abide by international agreements, including the Universal Declaration of Human Rights, which enshrines freedom of religion.
Abdul Rahman told a preliminary hearing last week he had become a Christian while working for an aid group helping Afghan refugees in Pakistan.
The Afghan judiciary is known to
be a bastion of conservatism
He was detained after his family informed authorities he had converted, apparently after a family dispute involving two daughters, a judicial official said.
A prosecutor has raised questions about Rahman's mental state, and a judge said that could be taken into account. Abdul Rahman has denied he is mentally unstable.
Britain, Australia, Germany, Italy, Sweden and Canada and the UN also voiced their concern, threatening to drive a rift between Afghanistan and the Western countries it relies on to rebuild after 20 years of war.
Condoleezza Rice, the US secretary of state, telephoned Hamid Karzai, the Afghan president, on Thursday to step up pressure to free Abdul Rahman.
Rice said she phoned Karzai to "hammer home in the strongest possible terms" Washington's concern over the proceedings against Abdul Rahman, while Bush said he was "deeply troubled" by the case.
The US has about 16,000 troops there helping to overpower Taliban and other anti-government insurgents, and is the main donor to the destitute country.
Rice phoned Karzai to express
US concern over the case
Stephen Harper, the Canadian prime minister, who has more than 2000 troops in Afghanistan, said he telephoned Karzai to express concern about a possible execution and "he conveyed to me that we don't have to worry about any such eventual outcome".
John Howard, the Australian prime minister, who has around 200 soldiers in Afghanistan, described the case as "appalling".
As the international outcry mounted, Afghanistan's Supreme Court said on Thursday it was trying to find a "good solution", including persuading Abdul Rahman to revert to Islam.