Qatar's prime minister is due to travel to Tehran on Thursday as Doha seeks to restart diplomatic efforts between Iran and the United States, amid rising tensions over Washington's decision to increase economic pressure on Tehran. Qatar has close ties with both sides: it is a U.S. ally but also neighbors Iran, and has previously acted as a communication channel between the two countries. Doha also helped secure a ceasefire in June that brought a temporary halt to the fighting. The visit comes as the conflict approaches its sixth month. Although major fighting has largely stopped, the two sides have yet to reach a lasting diplomatic settlement. One of the biggest unresolved issues is the Strait of Hormuz, the narrow waterway.