Pro-Russian separatists rejected a unilateral cease-fire declared by Ukraine's newly inaugurated president, casting doubt on the new effort to end the conflict in the country's east that has killed hundreds.
Ukraine's border guard service said rebels attacked one of its posts in the Donetsk region overnight, hours after President Petro Poroshenko ordered the halt in fighting. At least three guards were injured, the agency said.
The cease-fire announced Friday was the latest attempt to try to tamp down the fighting that has seen Russian-backed fighters seize towns and arsenals around Donetsk and eastern regions over the past two months.
Poroshenko's order told Ukraine's military and security forces to halt operations for seven days and urged rebels to lay down their arms.
The 14-point plan would set up a demilitarized zone along the Ukrainian-Russian border and provide an escape corridor for mercenaries that the Ukrainian government has said are in eastern Ukraine.
Moscow has sent mixed signals both about ending support for the separatists and backing a lasting peace deal.
Russian Foreign Minister Sergei Lavrov denounced the cease-fire as an "ultimatum." In a statement, the Kremlin said President Vladimir Putin supported Poroshenko's call.
"However, the Russian head of state drew attention to the fact that the proposed plan without practical actions aimed at the beginning of the negotiation process will not be viable and realistic," the Kremlin said.
Also Saturday, the Kremlin announced that Putin had put troops in central Russia on "combat alert" for snap drills. Russian officials say the drills involve some 65,000 troops, in a region that does not border Ukraine.
A NATO military officer told VOA Friday that the presence of new Russian troops near the border with Ukraine do not appear to be engaged in "border patrol" duties, but instead seem to be concentrating in staging areas and preparing and awaiting future orders.
He said the new troops were not "an encouraging sign."
Deputy White House spokesman Josh Earnest said Friday the United States is concerned about the buildup.
Kremlin spokesman Dmitry Peskov said the deployment had been planned in advance and was designed to reinforce Russia's border controls.
Ukraine and Russia have been locked in a standoff since late February when violent protests forced Viktor Yanukovych from the presidency and Russian forces moved into Crimea. Russia later annexed the Black Sea peninsula in March, and in April, the insurgency erupted in Ukraine's east.
VOA Pentagon correspondent Jeff Seldin contributed material to this report.