(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 didn'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.
CNN's Laura Smith-Spark and Victoria Butenko contributed to this report