Moscow - Russia's foreign minister said on Monday that a surge in fighting near Ukraine's south-eastern port city of Mariupol is a likely harbinger of a Ukrainian military offensive eastward into rebel-held territory."We are worried by the development of events in recent days that are very strongly suggestive of a preparation for further military actions," Foreign Minister Sergei Lavrov told reporters in Moscow, according to state news agency RIA Novosti.Lavrov said that Ukraine and the pro-Russian rebels had been negotiating for a withdrawal of weapons away from the area, including demilitarisation of the town of Shyrokyne, which is near Mariupol and has seen intense fighting.But Lavrov said that the negotiations have stalled because of inflexibility by the Ukrainian government and that the area separating the Ukrainian forces from the rebels can no longer be described as a "contact line", but rather a "front”.Several deaths, including at least two civilians, were reported by local media following a series of skirmishes in the region over the weekend.Ukrainian military spokesperson Andriy Lysenko said that two servicemen were killed and seven wounded in recent days due to "provocations" by the rebels.The European Union's foreign policy spokesperson, Catherine Ray, mentioned civilian casualties on Monday and urged the belligerents to comply with the February ceasefire."It is of utmost importance that the two sides fully observe the ceasefire and take all appropriate measures to protect civilians in full respect of international humanitarian law," Ray said."We condemn the recent escalations of fighting which continued this [past] night which resulted in the death of civilians in two cities," Ray added. She did not blame either side for the violence.Russia and Ukraine have long blamed each other for attacks around Ukraine's two eastern-most regions - Donetsk and Luhansk - which have been locked in a conflict between the Ukrainian military and pro-Russian rebel groups since spring 2014.Russia has adamantly denied Ukraine's allegations that it has sent soldiers to fight on behalf of the rebels. But Russia supports the separatist cause and has said it is using its influence over the rebels to push for peace in the region.According to estimates by the UN, more than 6 500 people have died in the conflict, which began soon after Ukraine's former pro-Russian president was ousted during mass protests that called for closer ties with the European Union.