Sept 30 (Reuters) - Twitter Inc TWTR.N said on Wednesday it was partnering with U.S. retailers to help them sell products through a "Buy Now" button in tweets, the latest move by the company to boost flagging user growth.

The "Buy Now" feature, which was initially rolled out to a small percentage of U.S. users last year, allows a customer to buy a product directly from a tweet.

The micro-blogging site said it was working with e-commerce websites such as Bigcommerce, Demandware Inc DWRE.N , and Shopify Inc SH.TO , and retailers and brands such as Best Buy Co Inc BBY.N , Adidas AG ADSGn.DE and PacSun.

Re/code reported on Tuesday that Twitter was building a new product, which would allow users to share tweets longer than its ubiquitous 140 characters. ID:nL3N11Z4WU

This comes at a time when Twitter is working overtime to boost user growth, after the company's second-quarter monthly average users grew at the slowest pace since the company went public in 2013.

The company has struggled with direct response ads this year, which are intended to encourage actions such as clicking on a link or downloading an app.

Twitter had already announced plans to allow users to make political donations through tweets with the help of Square, the mobile payment company headed by Twitter's interim Chief Executive Jack Dorsey.

The company has tied up with payment processor Stripe Inc to let shoppers use the "Buy Now" button.

(Reporting by Anya George Tharakan in Bengaluru and Yasmeen Abutaleb in San Francisco; Editing by Anil D'Silva) ((anyageorge.tharakan@thomsonreuters.com; within U.S. +1-646-223-8780; outside U.S. +91 80 6749 7118;; Reuters Messaging: Reuters Messaging: anyageorge.tharakan.thomsonreuters.com@reuters.net))