Ukraine's president on Monday announced a partial call-up of reservists for training amid tensions with Russia, saying the country needs to beef up its defenses to counter the threat of a Russian invasion.

The Kremlin dismissed the Ukrainian leader's statement as an "absurd" attempt to inflame tensions.

Relations between the two neighbors have been strained further following a November 25 incident in which the Russian coast guard fired upon and seized three Ukrainian naval vessels and their crews off the Crimean Peninsula that Russia annexed from Ukraine.

Ukrainian President Petro Poroshenko responded by introducing martial law for 30 days in much of Ukraine.

For the duration of martial law, Ukrainian authorities barred entry to all Russian males aged 16 to 60 in a move the Ukrainian leader said was needed to prevent Russia from further destabilising the country.

Poroshenko said on Monday that some reservists will be summoned for training as part of martial law. He also said that some military units will be redeployed to strengthen the nation's defenses.

"Ukraine is taking its own steps in response to the threat of a large-scale Russian invasion," the Ukrainian leader said.

Over the weekend, Poroshenko said that Russia has deployed a large number of troops along its border with Ukraine and alleged that the Kremlin intends to push inland into Ukraine.Ukraine also accused Russia of blockading its ports on the Sea of Azov.

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 that navigation has continued normally except for occasional breaks because of bad weather.

The separatist conflict in eastern regions forming Ukraine's industrial heartland has taken a toll on the national economy, reducing the cargo flow through the Ukrainian ports of Mariupol and Berdyansk on the Sea of Azov. The naval incident further stoked tensions.

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 Crimea were conducting drills involving Bal and Bastion long-range anti-ship missile systems.

Over the weekend, Poroshenko urged Germany and other Western allies to boost their naval presence in the Black Sea to help deter Russia from further aggression.

Ukraine's deputy defense minister, Anatoliy Petrenko, said the country is talking to its Western partners to respond to Russia's "escalatory actions".

The United States 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 on 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 set to discuss the incident later this week.

Asked what action Nato could take, Stoltenberg said: "We provide strong political but also strong practical support to Ukraine."

He added that Nato allies have helped modernize Ukraine's armed forces, have boosted their presence in the Black Sea over the last year, with more ships deployed in the region and more air policing.

In the Nov 25 incident, three Ukrainian naval vessels were heading from the Black Sea into the Sea of Azov when they were blocked by the Russian coast guard near the Kerch Strait between Russia's mainland and Crimea.

After many tense hours of maneuvering, the Russians opened fire and seized the Ukrainian vessels and 24 crew members.

The Ukrainian seamen have been put in custody for two months pending Russia's investigation into the clash.

Ukraine and Russia have traded blame for the naval incident that further escalated the tug-of-war that began in 2014 when Russian annexed Crimea and backed separatist rebels in eastern Ukraine.

Ukraine says that its ships were operating in line with a 2003 treaty with Russia envisaging free navigation for vessels of both countries in the Sea of Azov and the Kerch Strait.

Russia is adamant that the Ukrainian ships had entered its waters without permission.

Later this week, the Ukrainian parliament is expected to mull a presidential bill blocking the extension of a friendship treaty after it expires next April, a long-anticipated symbolic move.

Poroshenko has said he tried to arrange a phone call with Putin to discuss the standoff but faced the Kremlin's refusal.

Peskov said on Monday that "no such conversation is planned".