By Nandita Bose

CHICAGO (Reuters) - Wal-Mart Stores Inc said on Wednesday it has completed the rollout of its Wal-Mart Pay mobile payment service across the United States and that 88 percent of transactions on the payment app are from repeat users.

Overall transactions on the app, which the world's largest retailer launched in December, jumped 45 percent in the last week, Daniel Eckert, senior vice-president of services at Wal-Mart US, said on a conference call with the media.

Wal-Mart declined to disclose the increase in transactions since the launch, or the number of the mobile app's users in its stores.

U.S. retailers have launched many mobile payment apps in the last two years, but customers and merchants have been slow to adopt them.

U.S. mobile payments accounted for an estimated $67 billion of purchases in 2015, and are expected to grow this year to $83 billion, or 24 percent of all purchases made via smartphones, according to the latest Forrester Research data.

Eckert said Wal-Mart Pay users have not been spending more as a result of using the app. The company is monitoring shopping patterns to see if purchases would increase.

The retailer will start advertising the app to push customer usage, he said.

Walmart Pay is available on Apple and Android devices and allows payments with any major credit, debit, pre-paid or Walmart gift cards.

Customers at a checkout counter must choose the payment option within the app and use their smartphone camera to scan the code displayed at the register. An e-receipt would be sent to the app. Apple Inc's Apple Pay and Alphabet Inc's Android Pay require retailers to install compatible new equipment, which has hindered wider acceptance.

Wal-Mart does not accept external mobile wallets like Apple Pay in its stores. Discussions about accepting third party wallets are ongoing, but Wal-Mart has no immediate plans to do so, Eckert said.

Wal-Mart leads a consortium of U.S. retailers to develop a mobile wallet app called CurrentC. The group, which includes Target Corp and Best Buy Co Inc, said earlier this year it would delay launching the app after the project hit several roadblocks.

(Reporting by Nandita Bose in Chicago; Editing by Richard Chang)