Russia's Foreign Ministry has said "it is quite possible" that Ukraine "might switch to full-scale combat actions within the next few days."
Foreign Ministry spokeswoman Maria Zakharova made the statements in Moscow on December 24, repeating claims by Russian officials predicting a "provocation" by Kyiv in the near future.
The unsubstantiated claims come at a time of high tensions between the two countries since Russia seized three Ukrainian Navy ships and their crews near the Kerch Strait on November 25. The United States and the European Union have denounced that action and called on Russia to release the vessels and seamen immediately.
"Apart from a ground operation, Kyiv is also mulling other option to increase tensions in the region and extend martial law," Zakharova said. "We are really concerned about increasingly more frequent reports by 'experts' in Ukrainian media outlets on the possible use of chemical-warfare agents by [Russia-backed militants in eastern Ukraine]."
"This circumstance gives grounds to presume that the strategists in Kyiv have thoroughly analyzed the record of their foreign supervisors in staging provocations involving the use of weapons of mass destruction and are ready to put them into practice."
On December 1, Ukrainian President Petro Poroshenko said Russia was amassing ground forces and weaponry along the border between the two countries.
Speaking at a military event in Kyiv on December 1, Poroshenko said Russia has deployed "more than 80,000 troops, 1,400 artillery and multiple-rocket-launch systems, 900 tanks, 2,300 armored combat vehicles, 500 airplanes, and 300 helicopters" near the border.
On December 22, Russia moved more than a dozen advanced fighter jets to the annexed Ukrainian region of Crimea.
Ukrainian government forces have been fighting against Russia-backed separatists in the eastern Ukraine since April 2014, shortly after Russia seized Ukraine's Crimean Peninsula and forcibly annexed it. Some 10,300 people have been killed in the fighting since early 2014.
Although Moscow denies interfering in Ukraine's domestic affairs, the International Criminal Court (ICC) in November 2016 ruled that the fighting in eastern Ukraine is "an international armed conflict between Ukraine and the Russian Federation."