Twitter is now testing Fleet ads, which would bring vertical format, full-screen ads to the platform for the first time, the company announced Tuesday. Fleet ads support 9:16 aspect ratio video (up to 30 seconds long) and images, and feature a swipe-up call-to-action .

Fleet ads will appear between Fleets from other users or brands, similar to Instagram Stories ads, and they are currently only visible to a limited group of mobile users in the U.S.

Performance metrics. Twitter’s standard ads metrics (impressions, profile visits, clicks, site visits, etc.) are also available for Fleet ads. For Fleet ads that leverage video assets, Twitter will also show the number of total views, 6s video views [advertisers are charged after their video has been viewed for 6 seconds], starts, completes, and quartile reporting.

More full-screen ad formats may be coming to Twitter. “As we experiment on this new surface for ads, we’ll take a close look at how vertical, full-screen ads perform on Twitter,” the company said in its announcement, “We want to understand how this content performs for customers not just for Fleet ads, but for future iterations of full-screen formats on Twitter.”

It’s unclear what proportion of users have adopted Fleets, and it’s not uncommon for features to come and go by the wayside. Twitter’s statement, however, expresses an investment in full-screen ad formats beyond Fleet ads.