U.S. and European leaders threatened new sanctions against Moscow after a missile attack blamed on pro-Russian separatists killed 30 civilians in the eastern Ukrainian city of Mariupol, the latest escalation in violence that has brought Kiev's fight with rebels back toward full-scale war.

Russia reacted with defiance, blaming Kiev and its Western backers for the surge in fighting, but it also called for urgent talks on implementing a September cease-fire. Separatists backed off earlier threats of a broad offensive on Mariupol and other targets, but shelling along the contact line between the two sides was extremely heavy over the weekend, Ukrainian military officials said.

U.S. President Barack Obama said he was deeply concerned about the latest break in the cease-fire and escalating separatist aggression, saying he would continue to ratchet up pressure on Russia. "I will look at all additional options that are available to us short of a military confrontation in trying to address this issue," the president said at a Sunday news conference in New Delhi.

The European Union, saying the rebels "bluntly refuse to observe" the cease-fire, called an emergency meeting of foreign ministers for Thursday to discuss a response.

Diplomats said it isn't yet clear whether the West is unified enough to agree on substantial new sanctions against Russia. The latest explosion in violence came as a surprise, just as the EU had begun considering the conditions under which it could start to ease some limits on Russia. The U.S., meanwhile, is wary of taking major new steps without Europe's support.

Publicly, the Kremlin remains defiant, appearing confident that the Russian public will blame the West for the worsening economic pain caused by sanctions and the fall in oil prices. Kremlin insiders say the leadership is giving mixed signals on whether it is seeking to win an easing of sanctions by pressuring Kiev into a truce or preparing for further escalation.

At the World Economic Forum in Davos, Switzerland, last week, a top Russian official suggested the standoff with the West could last decades and said the government was looking at the experience of regimes that suffered years of international isolation, such as South Africa's apartheid government. Moscow's apparent resilience amid a 50% drop in the value of its currency and an economy pushed into recession has left Western capitals struggling to find effective levers to get the Kremlin to change direction.

"The question is going to be whether they continue to pursue a path where that not only is bad for the people of Ukraine but is bad for the people of Russia, and are we able to continue to raise the costs even as we're creating an off-ramp diplomatically that eventually the Kremlin starts pursuing a more sensible policy in resolving this issue," Mr. Obama said on Sunday.

The EU's foreign-policy chief, Federica Mogherini, warned on Saturday that further violence in eastern Ukraine "would inevitably lead to a further grave deterioration of relations between the EU and Russia."

But several senior EU diplomats on Sunday said there were no concrete ideas on the table yet for fresh Russia sanctions and the discussion of options was only starting.

The EU has already introduced restrictions on trade, defense and energy links with Russia and has targeted dozens of Russian officials and separatist leaders with a freeze on assets and travel restrictions.

Among the steps the EU and U.S. could take is widening the net of companies frozen out of Western financial markets to include more state-run firms or private companies. They could also ban exports of technology related to the gas industry in addition to measures against Russia's oil sector.

One measure that has been discussed previously is whether to seek to knock Russia off the so-called Swift network, which is key to making global financial transactions. In the past, diplomats have said that would likely be a last resort.

Any fresh measures must be backed by all 28 EU member countries and could face stiff resistance from Hungary, Cyprus and Greece, which have repeatedly expressed concerns about the measures.

Donald Tusk, president of the European Council and a former Polish prime minister, signaled he favored a tough response. "Once again, appeasement encourages the aggressor to greater acts of violence," read a post on his Twitter feed. "Time to step up our policy based on cold facts, not illusions."

Also on Saturday, the North Atlantic Treaty Organization accused Moscow of "supporting (the) offensive operations" of the separatists with heavy weapons. Russia has consistently denied such charges.

In separate phone calls on Sunday with Ms. Mogherini and U.S. Secretary of State John Kerry, Russian Foreign Minister Sergei Lavrov laid the blame for the latest surge in violence on Kiev, which he accused of violating the terms of the September cease-fire by shelling rebel-held cities, the ministry said. He called on his Western counterparts to push Kiev back into negotiations.

Ukrainian President Petro Poroshenko denied violating the cease-fire and said he remained committed to the September agreement.

But some in Kiev are wary that with rebels advancing, talks are likely only to cement separatist gains. Oleksandr Danyliuk, a senior official in the Poroshenko administration, said, "Any talks will create a new reality in which Ukraine will most likely be the loser. We can do nothing but resist in every way possible."

Rebels said over the weekend they were pulling out of cease-fire talks because they hadn't yielded results. Separatist leaders sent mixed signals on their military aims, however. After initially announcing after the Mariupol missile strike that a broad attack on the Ukrainian-held city was under way, top rebels reversed themselves and told Russian state television they weren't planning a siege.

Saturday's missile strike hit a residential area, killing 30 and wounding about 100. Monitors from the Organization for Security and Cooperation in Europe said the missiles came from rebel-held territory, though separatists denied responsibility.

Ukrainian officials said there was no sign of a major assault on Sunday on Mariupol, a strategic port city that has been largely spared from the fighting. A rebel push on Mariupol would be a major escalation, but taking the city would give the economically challenged rebels a much-needed seaport.

On Sunday, Ukrainian military spokesman Col. Andriy Lysenko said elsewhere on the front, rebels were "shelling our forces with extraordinary intensity using artillery, mortars, grenade-launchers, tanks and "Grad" missile launchers."

Heavy fighting was reported around Debaltseve, a Ukrainian-held transport hub northeast of the rebel capital of Donetsk. Rebel leaders said on Saturday that they aimed to surround Ukrainian troops there. Russian military observers in the city evacuated last week, citing security concerns, the OSCE said.

Military analysts say the rebels" latest push appears to be an effort to straighten out the jagged line dividing the two sides and make their self-declared state more secure. The separatists won a symbolic victory in that drive last week, forcing Ukrainian troops to retreat from the devastated Donetsk airport.

Colleen McCain Nelson and Matthew Dalton contributed to this article.

Write to Gregory L. White at greg.white@wsj.com and Laurence Norman at laurence.norman@wsj.com

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