African Union chief Moussa Faki Mahamat called on Saturday for measures against the key players in the conflict in South Sudan, saying "we are used to them not respecting their commitments"."The situation in South Sudan is serious. The humanitarian and security situation is increasingly difficult. The last few days we've had some good news, which I unfortunately now have to question," he said at an AU meeting in Nouakchott.Moussa Faki Mahamat was referring to an agreement reached on Wednesday by South Sudan's President Salva Kiir and his arch-foe Riek Machar, which appears already to be in jeopardy."We are now used to the key actors in South Sudan not respecting their commitments... the situation is intolerable... It is time to act, to accept our responsibility," he said."We have to send a clear message to the players to respect their commitments," he added, warning of possible sanctions.President Salva Kiir and Machar agreed, at talks in the Sudanese capital Khartoum on Wednesday, to a "permanent" ceasefire to take effect within 72 hours, raising hopes of an end to four-and-a-half years of war.But South Sudan rebels, led by former vice president Machar, on Saturday accused the army of breaking the ceasefire just hours after it came into force.The conflict erupted in 2013, around two years after South Sudan won independence from Sudan, when Kiir accused his then-deputy Machar of plotting a coup.It has claimed tens of thousands of lives, displaced four million people and left the newly created country's oil-rich economy in tatters.Kiir and Machar signed the document - called the "Khartoum Declaration" -- in the presence of Sudanese President Omar al-Bashir.The United Nations had given the two South Sudanese sides until the end of June to find a viable political agreement or face sanctions.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.