The United States and European allies warned Russia not to send forces into Ukraine on Sunday as rival neighbours east and west of the former Soviet republic said a power vacuum in Kiev must not let the country break apart.
A day after Moscow-backed president Viktor Yanukovich fled to the Russian-speaking east following dozens of deaths on the streets, parliament named its new speaker as interim head of state. An ally of the ousted leader's long jailed rival Yulia Tymoshenko, he aims to swear in a government by Tuesday that can provide authority until a presidential election on May 25.
With battle-hardened, pro-Western protesters in control of central Kiev and determined to hold their leaders to account, lawmakers rushed through decisions to cement their power, display their rejection of rampant corruption and bring to book officials who ordered police to fire on Independence Square.
But whoever takes charge as interim prime minister faces a huge challenge to satisfy popular expectations and will find an economy in deep crisis, even if the EU makes good on new offers of aid that may help make up for loans that Russia has frozen.
Scuffles in Russian-speaking Crimea and some eastern cities between supporters of the new, pro-EU order in Kiev and those anxious to stay close to Moscow revived fears of separatism that a week earlier were focused on the west, where Ukrainian nationalists had disowned Yanukovich and proclaimed self-rule.
President Barack Obama's national security adviser, Susan Rice, was asked on U.S. television about the possibility of Russia sending troops to Ukraine, which President Vladimir Putin had hoped Yanukovich would keep closely allied to Moscow.
"That would be a grave mistake," Rice said. "It's not in the interests of Ukraine or of Russia or of Europe or the United States to see a country split. It's in nobody's interest to see violence return and the situation escalate."
Yanukovich's flight into hiding left Putin's Ukraine policy in tatters, on a day he had hoped eyes would be on the grand finale to the Sochi Olympics. The Kremlin leader spoke on Sunday with German Chancellor Angela Merkel, whose foreign minister had brokered a short-lived truce in Kiev on Friday.
They agreed Ukraine's "territorial integrity" must be maintained, Merkel's spokesman said in a statement.
British Foreign Secretary William Hague was asked if Russia might "send in the tanks" to defend its interests among ethnic Russians in the east and on the Crimea peninsula, where Moscow bases its Black Sea Fleet: "It would really not be in the interests of Russia to do any such thing," he told the BBC.
Earlier this month, a Kremlin aide warned that Moscow could intervene and accused Washington of breaching their 1994 treaty under which Russia removed Soviet nuclear weapons from Ukraine.
It is unlikely the United States and its allies in NATO would risk an outright military confrontation with Russia but such rhetoric, laden with echoes of the Cold War, underlines the high stakes in Ukraine, whose 46 million people and sprawling territory are caught in a geopolitical tug of war.