( CNN ) -- The new cease-fire implemented in embattled eastern Ukraine by Ukrainian President Petro Poroshenko on Friday was clearly under threat by Saturday , if not sooner . Three Ukrainian border guards were wounded when pro-Russian separatists launched a mortar attack and opened fire with automatic weapons near the town of Vyselky , in the Donetsk region , Ukraine 's State Border Service said in a statement Saturday . The wounded guards were evacuated as security forces were notified , according to the Border Service . The separatists also targeted a nearby checkpoint in Uspenka , using small arms , but the attack was repelled by guards , the Border Service said . Russia and Ukraine have been engaged in a tense standoff since March , when Russia annexed Crimea and massed troops along its border with Ukraine . Ukraine 's government in Kiev has accused Russia of allowing weapons and military equipment to cross the border illegally into the hands of pro-Russia separatists , who have led uprisings in Ukraine 's eastern Donetsk and Luhansk regions . Meanwhile , three military tanks and 10 armored personnel carriers of unknown origin were seen in Luhansk , the Organization for Security and Co-operation in Europe , which operates a special monitoring mission in eastern Ukraine , said Saturday . The military convoy drove through the city early Friday morning , according to local witnesses and traffic surveillance camera footage seen by the mission . The convoy came from the direction of the Russian border , going east and was observed in Alchevsk , 27 miles southwest of Luhansk , heading toward Donetsk , OSCE said . Meanwhile , four OSCE monitors from the Donetsk team have been missing for 26 days and four other monitors from the Luhansk team for 23 days , the group said . Kremlin responds to cease-fire plan . Also Saturday , Russian President Vladimir Putin expressed support for Poroshenko 's cease-fire plan , the Kremlin said in a statement . Putin emphasized that the plan `` should not take the character of an ultimatum to militants '' and that the cease-fire `` should be used to begin constructive negotiations and political compromises between the opponent parties in eastern Ukraine , '' according to the Kremlin . But the statement also mentioned an incident Friday on the Ukrainian-Russian border after the cease-fire was first declared . The Russian Foreign Ministry said one of its units in Novoshakhtinsk , near the border with Ukraine , was shelled , injuring one customs officer . Ukraine denied that it fired artillery or mortar at the Russian unit . Putin said in the statement released Saturday that he considers the incident `` unacceptable , '' especially because it happened after Poroshenko declared a cease-fire . Poroshenko declares cease-fire . Poroshenko declared a cease-fire Friday in Kiev 's fight against separatists in the country 's pro-Russian east . He said military units would respond only if they 're attacked and he warned militant fighters to abandon their weapons before the cease-fire expires on June 27 . `` Those who will not lay down their weapons will be destroyed , '' Poroshenko said , according to a statement issued by the Interior Ministry . Poroshenko announced plans for the cease-fire on Wednesday . He spoke with Putin by phone on Thursday , according to Poroshenko 's office . Putin has said he backs an effort to de-escalate tensions in eastern regions and supports a process to forge a cease-fire and a peace plan . President : Free hostages . Poroshenko 's cease-fire includes the closure of the Ukraine-Russia border and changes to the constitution to decentralize power . It also offers amnesty to those who did n't commit serious crimes , the President said . An escape corridor will be offered for those who disarm to leave Ukraine . `` We expect that hostages and seized premises will be liberated . We expect that a large number of civilians will use the security guarantees for the citizens of Donbas , '' Poroshenko said , referring to Ukraine 's eastern region . In his conversation with Putin , Poroshenko stressed the need for the release of Ukrainian hostages and to establish effective security controls on the border with Russia . Ukraine implements cease-fire in restive east . Putin , Ukraine leader talk peace in phone call . Ukrainian military plane shot down ; all 49 aboard dead . CNN 's Laura Smith-Spark and Victoria Butenko contributed to this report .
Violence erupts amid attempts at new cease-fire in eastern Ukraine . Pro-Russian separatists attacked with mortars , weapons fire , Ukrainian officials say . Ukrainian guards repelled a separate attack at a checkpoint nearby . Kremlin statement says Putin supports a cease-fire .