KIEV, April 14 - Pro-Russian separatists on Monday ignored an ultimatum to leave occupied government buildings in eastern Ukraine and instead seized more buildings as the government failed to follow through on a threatened military crackdown, Reuters reported.

Pro-Russian rebels have seized buildings in around 10 other towns and cities across other eastern provinces that form the heartland of Ukraine's heavy industry.

In the town of Slavyansk, where the authorities failed to follow through with their announced "anti-terrorist" operation, rebels called for Putin's help.

The Kremlin said the Russian president was listening.

"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," spokesman Dmitry Peskov said. "The president is watching the developments in Eastern Ukraine with great concern."

The Ukraine crisis, and in particular the annexation of Crimea by Russia last month, have led to the most strained relations between Moscow and the West since the Cold War.

European foreign ministers agreed to step up sanctions, and U.S. officials have said they were in consultations with European partners on how to punish Moscow for what Kiev and its Western allies call a Russian plot to dismember Ukraine.

Moscow has largely brushed off sanctions so far, which the United States and Europe have explicitly designed to target only a limited number of officials and avert wider economic harm.

Ukraine's interim president, Oleksandr Turchynov, said on Monday the offensive against the rebels would still go ahead. But in a sign of discord behind the scenes in Kiev, he sacked the state security chief in charge of the operation.

In one of the first signs of a military deployment by Kiev's forces, a Ukrainian column of two tanks and more than 20 armored personnel carriers packed with paratroops was seen about 70 km (50 miles) northwest of Slavyansk on Monday evening, according to video journalist Maksim Dondyuk who filmed them.

In Donetsk, rebels holed up in the administrative headquarters of a province that is home to 10 percent of Ukraine's population said they planned to seize control of infrastructure and the levers of state power. They have declared an independent "People's Republic of Donetsk" and sought Putin's protection if they are attacked.

In a bid to undercut the rebels' demands, Turchynov held out the prospect of a countrywide referendum on the future shape of the Ukrainian state. Pro-Russian secessionists want separate referendums in their regions, which Kiev says is illegal.

The uprising in eastern Ukraine began eight days ago but has accelerated sharply in the past 48 hours, with separatists seizing ever more buildings, including arsenals filled with weapons. They have met little opposition.

Kiev says the separatists are organized by Moscow, seeking to repeat the seizure of the Crimea region.

Russia says the armed men are all locals acting on their own, but Western officials say the uprising is too well-coordinated to be entirely spontaneous, and bears too many similarities to the Russian operation in Crimea.

Moscow says it has the right to intervene to protect Russian speakers in Ukraine, and has portrayed the people of the east as under threat from gangs of Ukrainian-speaking "fascists.”

NATO says Russia has tens of thousands of troops massed on the frontier, able to capture eastern Ukraine within days. (rt/ez)