Washington has imposed export licensing restrictions on 15 key oil organizations in South Sudan, in an effort to apply pressure on the government to engage constructively in talks to bring an end to the civil conflict that has engulfed the country since December 2013.

The measure, introduced by the Department of Commerce on 21 March, requires that companies providing US equipment and services to firms on the list must first obtain a government licence. Juba earns almost all its revenue from...