U.S. Secretary of State John Kerry on Tuesday accused Russian agents of stirring unrest in eastern Ukraine that could be used as a pretext for military intervention.

"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 engaged in this initiative," Kerry told the Senate Foreign Relations Committee.

Pro-Russia activists seized several government buildings in eastern Ukraine Sunday, demanding closer ties with Moscow and a referendum to give more powers to local authorities.

The Ukrainian parliament approved a bill against separatism on Tuesday in a bid to end violent pro-Russian rallies in the country 's east regions.

"Russia's clear and unmistakable involvement in destabilizing and engaging in separatist activities in the east of Ukraine is more than deeply disturbing," Kerry said.

"No one is fooled by what could potentially be a contrived pretext for military intervention, just as we saw in Crimea," the top U.S. diplomat said.

Russia has rejected the accusation that it is involved in fomenting chaos in eastern Ukraine.

"Stop blaming Russia for all of Ukraine's problems. Ukrainian people want to hear meaningful answers from official Kiev," the Russian Foreign Ministry said in a statement posted on its website.

While reiterating that the United States and its allies prefer a diplomatic solution to the crisis, Kerry expressed their willingness to put in effect tough new sanctions on the Russian economy, including on key sectors such as energy, banking and mining.

"It doesn't have to be this way. But it will be this way if Russia continues down this provocative path," he said.

In a telephone talk between Kerry and Russian Foreign Minister Sergei Lavrov on Monday, they agreed to hold talks next week in Europe with Ukrainian and European officials over the Ukraine crisis.

On Tuesday, Lavrov reiterated Moscow's readiness for the four- party talks and said a draft new constitution of Ukraine could be considered during the talks.

The Ukrainian Foreign Ministry also said Kiev stands ready for dialogue with Moscow in any format to find a way out of the ongoing crisis.

At the Senate hearing on Tuesday, Kerry urged Russia to take concrete steps to disavow separatist actions in eastern Ukraine, pull back its forces outside the country and demonstrate that it is "prepared to come to these discussions."

"Russia has a choice: to work with the international community to help build an independent Ukraine ...or they could face greater isolation and pay the costs for their failure," Kerry said.