Twitter is rolling out self-serve advertising to small businesses through promoted tweets and promoted accounts, the company announced in a blog post.

Starting Monday, eligible American Express cardmembers and merchants will be able to start advertising via Twitter. To participate, they must first register with Twitter here.

Earlier this month, Twitter launched a partnership with American Express that offers cardholders special offers and discounts through certain hashtags. Those who are interested must first sync their AmEx cards with Twitter.

Rolling advertising out to small businesses is an extension of the existing partnership with American Express, as well as a part of Twitter's recent advertising push. Last week, Twitter announced that it will expand the availability of promoted tweets and accounts on mobile. These paid-for items first rolled out to Twitter's iOS and Android apps in February.

At launch, promoted tweets were only seen by users who already followed the brand posting the message. However, going forward, Twitter is allowing brands to send promoted tweets to users who share interests with the brand's existing followers.

Among the benefits of promoted tweets and accounts for small businesses are ongoing management of ads, the addition of new followers, the ability for the brand to reach more people, and the option to target specific countries, cities or other geographic areas.

Twitter is believed to be preparing an overhaul of brand pages that would allow advertisers to add on experiences onto the pages, such as e-commerce, contests, and sweepstakes.

Twitter last week celebrated its sixth birthday. The microblog service now boasts 140 million active users that send 240 million tweets each day.