The Kremlin dismissed the Ukrainian leader's statement as an "absurd" attempt to inflame tensions. Ukraine has also accused Russia of blockading its ports on the Sea of Azov and urged Germany and other Western allies to boost their naval presence in the Black Sea to help deter Russia from further aggression.
Relations between the two neighbors have been strained following a Nov. 25 clash in which the Russian coast guard fired upon and seized three Ukrainian naval vessels and their crews off the Crimean Peninsula, which Russia annexed in 2014 from Ukraine.
Ukrainian President Petro Poroshenko responded by introducing martial law for 30 days in the country's border areas and barring all Russian males aged 16 to 60 from entering the country, a move he said was needed to prevent Russia from assembling destabilizing forces in Ukraine.
Poroshenko said Monday that some reservists will be summoned for training as part of martial law and some military units will be redeployed. "Ukraine is taking its own steps in response to the threat of a large-scale Russian invasion," he said.
Over the weekend, Poroshenko said Russia has deployed a large number of troops along its border with Ukraine and alleged that the Kremlin intends to push inland into Ukraine. Russian President Vladimir Putin's spokesman, Dmitry Peskov, dismissed Poroshenko's claims as an "absurd attempt to foment tensions."
"The accusations against Russia have no basis whatsoever," he said. Peskov also rejected Kiev's claim that Russia was blocking traffic to and from Ukrainian ports on the Sea of Azov, saying navigation has continued normally except for occasional breaks because of bad weather.
The separatist conflict in Ukraine's eastern industrial heartland against Russia-backed rebels has taken a toll on the nation's economy, reducing the cargo flow through the Ukrainian ports of Mariupol and Berdyansk on the Sea of Azov. The naval clash further stoked tensions — and 24 Ukrainian seamen are still in Russian custody in Moscow.
Vitaliy Sinhur, a dock worker in Berdyansk, said the movement of ships has significantly ebbed. Amid the tensions, the Russian military said its forces in occupied Crimea were conducting drills involving Bal and Bastion long-range anti-ship missile systems.
Ukraine's deputy defense minister, Anatoliy Petrenko, said the country is talking to its Western partners to respond to Russia's "escalatory actions." The U.S. and its NATO allies have strongly urged Russia to free the Ukrainian vessels and the crews.
"There is no justification for this use of force," NATO Secretary-General Jens Stoltenberg told reporters Monday. "Russia must release the Ukrainian sailors and ships. It must also allow freedom of navigation and unhindered access to Ukrainian ports in the Sea of Azov."
NATO foreign ministers are to discuss the incident later this week. Stoltenberg added that NATO allies have helped modernize Ukraine's armed forces and have boosted their presence in the Black Sea over the last year.
Russia is adamant that the Ukrainian ships had entered its waters without permission, while Ukraine says its ships were operating in line with a 2003 treaty with Russia that gives both nations free movement in the Sea of Azov and the Kerch Strait.
In the Ukrainian port of Mariupol, Ukrainian border guard spokesman Artem Polyakov said tensions in the area were boiling. "The Russian Federation is building up military forces in the Sea of Azov," he said.
Poroshenko said he tried to arrange a phone call with Putin to discuss the standoff but the Kremlin has refused. Peskov said Monday that "no such conversation is planned."
Vladimir Isachenkov reported from Moscow. Lorne Cook in Brussels and Mstyslav Chernov in Mariupol, Ukraine contributed to this report.