"The credit card number corresponds to a Barclay's bank (card) belonging to Annetta Flanigan," a Western official close to the investigation said on Saturday.
Annetta Flanigan is the British-Irish woman who was abducted at gunpoint along with a Filipino man and a woman from Kosovo in Kabul on Thursday.
The three were employed by the United Nations to help oversee Afghanistan's first presidential election on 9 October.
A splinter group of the Taliban, Afghanistan's former rulers who were toppled by US-led forces in late 2001, on Saturday issued two numbers to several media organisations which they said were the identity card numbers belonging to two of the captives.
The first turned out to be Flanigan's credit card number, while
the second has not yet been independently verified.
Jaish al-Muslimin (Army of Muslims), which says its followers
include former Taliban fighters, has distanced itself from the
Taliban movement led by Mulla Umar.
Mulla Muhammad Ishaq, who has been speaking on behalf of the group, said the captives were still safe, but would be executed unless foreign troops were withdrawn from Afghanistan.
They have also threatened to execute the captives if security forces searching for them use force to try to rescue them.