The latest round of high stakes nuclear talks between Iran and the US have ended, with Tehran calling them difficult but with both sides agreeing to further negotiations.

The talks seek to limit Iran’s nuclear program in exchange for the lifting of some of the economic sanctions the U.S. has imposed.

Though Tehran and Washington both have said they prefer diplomacy to resolve the decades-long dispute, they remain deeply divided on several red lines that negotiators will have to circumvent to reach a new nuclear deal and avert future military action.