The service, dubbed beba, is being rolled out initially in Nairobi. Users need a gmail account to activate the cards, which can be loaded up to a maximum value of $115 from a network of local agents.

Google is using Top Image - the same company that markets Safaricom's wildly successful M-Pesa - to market the beba cards.

Conductors on the bus network in Kenya tap the beba cards on an NFC-enabled mobile phone to process the payment, and the user gets a receipt by SMS. The beba Wesbite offers a full transaction history.

The cards may find a niche as they protect users from random fare hikes by unscrupulous bus conductors who pocket the cash paid on the fare.