Ukraine announced on Thursday that it will return to mandatory conscription in the coming months as its troops battle pro-Russian insurgents in the east.

"The National Security and Defense Council has decided to restart conscription in the fall (autumn)," Mykhailo Koval, deputy head of the council, said after an emergency meeting chaired by President Petro Poroshenko.

U.N. Secretary General Ban Ki-moon is alarmed by reports of fighting in east Ukraine spreading southward and warned of a dangerous escalation, his spokesman said Thursday.

The United Nations is unable to confirm reports that hundreds of Russian troops had crossed into Ukraine to shore up pro-Moscow rebels, the spokesman added.

U.S. Army troops equipped with tanks will head to Eastern Europe soon to reassure NATO allies anxious over Russia's intervention in Ukraine, Pentagon officials said Thursday.

About 600 troops from the 1st Brigade, 1st Cavalry Division are due to deploy in October to Poland and the Baltic states for training exercises with alliance members, replacing paratroopers from the 173rd Airborne Brigade, said spokeswoman Lieutenant Colonel Vanessa Hillman.

Italy's Prime Minister Matteo Renzi, whose country currently holds the rotating presidency of the EU, called Russian President Vladimir Putin on Thursday to warn of "very serious consequences" after reports that Moscow had sent troops into Ukraine.

The premier said the "entry of Russian troops into Ukraine" is "an intolerable escalation which will lead to very serious consequences".

Shelling in Ukraine's rebel-controlled city Donetsk has killed 15 since midday Thursday, local authorities said, describing the situation as "extremely tense."

Artillery fire in districts to the west, north, and northeast of the center destroyed infrastructure, including residential houses, the city hall said.

Ukraine's ambassador to the OSCE accused Russia Thursday of a "direct invasion" but Moscow's envoy denied reports that Russian troops were active in the conflict-torn east of the country.

"We registered a direct invasion by the Russian military into the eastern regions of Ukraine," Kiev's envoy Ihor Prokopchuk told journalists following a special meeting of the European security body to discuss the latest developments in Ukraine.

A leading volunteer commander in Ukraine triggered protests against the government on Wednesday after a series of angry Facebook messages over the failure to support troops besieged by rebels and rapidly running out of ammunition.

Pro-Russian rebels have surrounded the town of Ilovaysk, a railway hub 10 kilometers (six miles) east of their main stronghold of Donetsk.

Switzerland on Wednesday added 11 names and five banks to its blacklist over the Ukraine crisis in fresh moves to stop the country from being used to avoid EU sanctions against Russia.

The government in a statement said it had taken "all the measures required to ensure that the most recent sanctions imposed by the European Union cannot be circumvented via Swiss territory".

With munitions dumped in the center of town and military vehicles left by the roadside, the town of Starobesheve has the air of a place abandoned.

Situated on the road between the rebel stronghold of Donetsk and the Azov Sea to the south, the town bore all the signs on Wednesday of a hasty retreat by the Ukrainian army.

Poland's prime minister said Wednesday there is evidence that regular units of the Russian army are operating in Ukraine, according to information from NATO and Polish intelligence.

"No one can take seriously talk of separatists in Ukraine at this point," Prime Minister Donald Tusk told parliament regarding the crisis in neighboring Ukraine.