Isolated incidents of violence occurred Saturday in the eastern Ukrainian region of Donetsk, the Interfax news agency reported, but the ceasefire extension agreed to by the government and separatists appeared to be holding, according to dpa. Miroslav Rudenko, a separatist leader in the self-proclaimed Donetsk People's Republic, claimed there was military action in the city of Kramatorsk. President Petro Poroshenko's office announced early Saturday that the ceasefire had been extended until 1900 GMT Monday. The pro-Russian rebels also agreed to the truce after discussions with mediators representing the government. But Rudenko alleged that the ceasefire was aimed at giving the military time to strengthen its positions against the separatists. Pro-government forces in turn accused the rebels of shooting at soldiers at Kramatorsk airport. A military analyst in Kiev said there were no casualties. The 28 leaders of the European Unin on Friday issued an ultimatum to Russia and warned of fresh sanctions if there was no progress by Monday on de-escalating the violence. The EU's demands are: the return to Ukrainian control of three border posts held by separatists; the release of all separatist-held hostages, including international observers; the start of substantial negotiations on a peace plan; and agreement on a mechanism to monitor the ceasefire and border controls. The leaders said they "regret tat the ceasefire, while being respected by the Ukrainian authorities, has not led to the full cessation of military hostilities." The ceasefire, which began June 29 and had been set to expire late Friday, is the first step in a 15-point peace plan outlined by Poroshenko last week. It has been eagerly supported by the West. The separatists are demanding the withdrawal of all government troops before peace talks begin and have refused to lay down their arms.