Russian President Vladimir Putin is "very worried" about events in eastern Ukraine, where pro-Russian militants are occupying government buildings in defiance of a warning from Kiev to stand down.
The Kremlin has responded to an request for help from a separatist leader in Slavyansk, where at least two people were killed in clashes with Ukrainian forces on Sunday.
Mr. Putin's spokesman Dmitry Peskov said "Unfortunately, there's a great many such appeals coming from the Eastern Ukrainian regions addressed directly to Putin to intervene in this or that form".
"The President is watching the developments in eastern Ukraine with great concern".
The comment echoes language used ahead of the annexation of Crimea last month, when the Kremlin said appeals from ethnic Russians "would not be ignored" after key buildings were seized by pro-Moscow "self-defence" forces.
Armed men that Western leaders claim are Russian forces have set up checkpoints and barricades in towns across eastern Ukraine. The latest attack saw at least 100 armed men enter a police station in Horlivka on Monday, forcing riot officers to withdraw from the area.
The militants were seen smashing windows, ransacking the building and building barricades in footage beamed around the world via a live stream online.
Ukrainian TV footage showed an ambulance treating people who were apparently injured during the assault on the police HQ in the city of 300,000 people.
There was no immediate response from Ukrainian forces after acting President Oleksandr Turchynov said a "large-scale anti-terrorist operation" would be launched and the Donbass region would "soon be stabilised".
Mr. Turchynov's office said he had asked for peacekeeper help, telling UN Secretary General Ban Ki-moon "We are not against and welcome if, with your assistance, we conduct a joint anti-terrorist operation in the east".
No objection to a referendum
A peacekeeper deployment would have to be authorised by the UN Security Council, in which Russia holds a veto. But the interim government in Kiev did receive a boost when the US signed a US $1 billion loan guarantee designed to help Ukraine's ailing economy.
President Turchynov said earlier that he had no objection to a referendum in the east of the country to run alongside planned presidential elections as he believes the majority of Ukrainians would support an "independent, democratic and unitary Ukraine".
World leaders warned the violent seizure of the government buildings was a "dangerous escalation" in the crisis
The UK ambassador to the UN, Mark Lyall-Grant, told the Security Council that satellite images show between 35,000 and 40,000 Russian troops are massed near the Ukraine border, in addition to the 25,000 "illegally" in Crimea.
He said there were increasing signs of Russian involvement in orchestrating the violence.
"We want to use this Security Council meeting to expose that but also warn Russia against using events in eastern Ukraine as a pretext for further military escalation in the region" he said.
Russian ambassador Vitaly Churkin told the meeting he is alarmed by Ukraine's announcement of a "full-scale anti-terror operation" to seize back occupied areas. He denied Western claims that Moscow is behind the violence, and said Kiev has been using neo-Nazi forces to destabilise its eastern region.
"It is the West that will determine the opportunity to avoid civil war in Ukraine," he said.
"Some people, including in this chamber, do not want to see the real reasons for what is happening in Ukraine and are constantly seeing the hand of Moscow in what is going on. Enough. That is enough."
He said Russian-speaking people in eastern Ukraine are "concerned about their future" and "don't want radicals to impose their will on them".
Russian Foreign Minister Sergei Lavrov later demanded explanations after claiming there were reports the head of the CIA was visiting Ukraine.
European Union Foreign ministers are holding talks to discuss how to toughen sanctions against Russia without losing the support of EU governments worried about Moscow switching off the gas to Europe.
On his way into the meeting, Tanaiste and Foreign Affairs Minister Eamon Gilmore said the EU will consider a number of issues - including introducing more sanctions.
