Speaking before the Senate Foreign Relations Committee in Washington on Tuesday, US Secretary of State John Kerry warned that Russian President Vladimir Putin was deliberately trying to destabilize the interim government in Ukraine.

"What we see from Russia is an illegal and illegitimate effort to destabilize a sovereign state and create a contrived crisis with paid operatives across an international boundary," Kerry said.

On Sunday, pro-Russia protesters seized official buildings in three eastern Ukrainian cities – Kharkiv, Donetsk and Luhansk. The transitional leadership in Kyiv has contended that Moscow orchestrated the takeover in a bid to instigate unrest and prompt another secession from Ukraine, similar to Crimea's, which resulted in the Black Sea Peninsula joining the Russian Federation last month.

"Everything that we've seen in the last 48 hours, from Russian provocateurs and agents operating in eastern Ukraine, tells us that they've been sent there determined to create chaos," he said.

The demonstrations were a "contrived pretext for military intervention just as we saw in Crimea."

Kerry is scheduled to meet with his Russian counterpart, Sergey Lavrov, in Brussels next week.

The United States and the European Union have sought to hold talks with Russia over the crisis. However, Moscow has repeatedly eschewed attempts to deescalate tensions, which stem from events that led to Crimea's secession beginning in February. In the latest developments, Russian Foreign Minister Lavrov has demanded that the Russian-speaking southern and eastern regions of Ukraine be allowed to take part in the international dialogue. He has also denied any deliberate attempt on Russia's part to influence the Ukrainian people.

Kerry also warned on Tuesday that Washington was prepared to levy harsher sanctions on Moscow if it failed to comply with Western demands to stand down. The threat followed just hours after NATO Secretary-General Anders Fogh Rasmussen warned of "grave consequences" for Russia if it did not heed international demands to withdraw its troops from its border with Ukraine.

Hostage situation unclear

Meanwhile, Ukraine's state security service reported on Tuesday that protesters had taken 60 people hostage in a government building in Luhansk. They were reportedly using weapons and explosives to keep the hostages from leaving.

However, a man from the group denied the report in an interview with Reuters news agency.

"There are no explosives, no hostages. We do not need hostages to get what we want," the protester, who was identified as Anton, said.

Demonstrators in eastern Ukraine have demanded referendums be held similar to the one that Crimea held following the downfall of the government in Kyiv which included the ouster of Russia-backed President Viktor Yanukovych. The most recent unrest has resulted in clashes between the pro-Russia and pro-Kyiv supporters.

Ukrainian authorities have been able to reclaim buildings in Kharkiv, where they said on Tuesday that they had arrested 70 people during the operation.

kms/hc (AP, AFP, Reuters, dpa)