REPUBLIKA.CO.ID, WASHINGTON -- US Secretary of State John Kerry on Tuesday pushed Russia to stop backing pro-Moscow insurgents in Ukraine, as the West mulled further sanctions against the government of Vladimir Putin.

In a call to Russian Foreign Minister Sergei Lavrov, Kerry "expressed strong concern about the refusal of Russian-supported separatists to take the necessary steps" to enable the extension of a shaky 10-day ceasefire.

Kerry "stressed the importance of taking steps to de-escalate," and reiterated Washington's support for Kiev, a State Department spokesperson said in a statement.

He "also made clear that we and our European and international partners will continue to press Russia to end all support and weapons flowing to separatists," and do more to police the porous border, the statement said.

Washington also wants Moscow to "call on separatists to lay down their arms, to return the border checkpoints they hold to Ukrainian government control, and to release all remaining hostages."

Ukrainian tanks and fighter bombers resumed their assault on pro-Russian insurgents on Tuesday after Kiev's Western-backed leader brushed off a last-gasp European effort to save the tenuous truce.

Kiev and its Western allies have accused Putin of helping to arm and fund the separatists in reprisal for the February ouster of a Kremlin-backed leader who had rejected closer European ties.

The Kremlin denies those claims but still faces the threat of devastating sanctions against Russia's financial and energy sectors should Putin fail to demonstrate a clear desire to resolve the conflict.

Kerry and Lavrov also discussed the "grave security situation on the ground in Iraq" and the talks on Iran's nuclear program between world powers and Tehran that will resume later this week, the spokesperson said.

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