Violence in Ukraine escalated sharply Tuesday, as artillery shells and airstrikes pierced the relative calm of a 10-day cease-fire hours after President Petro Poroshenko allowed it to expire.

Both sides appeared to be readying for a protracted battle after days in which the fighting diminished but did not disappear. It remained unclear whether the Ukrainian military, which has battled pro-Russian separatists since mid-April, would be able strike a decisive blow against the rebels, who have seized territory in eastern Ukraine.

The longer a conflict drags on, the greater the risk of further civilian casualties and the harder it will be for Ukraine’s new government to stitch the society back together. Ukraine’s economy presented a formidable challenge even without a growing insurgency in the country’s industrial heartland. The foreign ministers of Ukraine, Russia, Germany and France planned to meet Wednesday in Berlin in a last-ditch effort to restart negotiations, the Russian Foreign Ministry said late Tuesday.

“The active phase of the counterterrorism operation resumed this morning,” the chairman of Ukraine’s parliament, Oleksandr Turchynov, told legislators Tuesday. “Our armed forces are hitting the bases and outposts of terrorists.”

Russian President Vladimir Putin condemned the renewed violence, putting Ukraine in a line with Iraq, Syria and Libya and saying that he could not stand by as a nation with close historical ties to Russia descended into chaos.

“All of us in Europe need a sort of safety net, so the Iraqi, Libyan, Syrian and Ukrainian precedents will not turn into an infectious disease,” Putin told a Moscow meeting of Russia’s ambassadors.

Poroshenko now bears full responsibility for the military operations, which started before he took office, Putin said, indicating that the period of relative conciliation from the Kremlin that started after Poroshenko’s May 25 election was at an end.

“We failed to convince him that the road to a secure, stable and inviolable peace cannot lie through war,” Putin said. He stopped short of any specific commitments about what Russia would do next.

Ukrainian officials have accused Russia of publicly pushing peace while quietly allowing weaponry and volunteers to flow over the border into eastern Ukraine, a charge the Kremlin has denied. Many of the key separatist leaders in Ukraine are Russian citizens, and rebels have said they have used advanced, Russian-made shoulder-mounted rockets to shoot down Ukrainian military aircraft.

Poroshenko had two long telephone conversations with Putin and the leaders of France and Germany in recent days in an effort to find a durable peace, but he had come under increasingly heavy criticism at home for the cease-fire, which never quelled the violence entirely. At least 27 soldiers died during the truce, the Ukrainian Foreign Ministry said, and many critics in Kiev said the pause in hostilities was giving separatists a chance to regroup.

“Every day of truce . . . strengthened the position of the terrorists from a military perspective,” Dmitro Tymchuk, an analyst close to the Ukrainian military, wrote on Facebook.

Scattered violence

Security forces and rebels skirmished across a wide swath of eastern Ukraine on Tuesday. In one victory for the government, security forces retook a contested border checkpoint, Poroshenko’s office said. Combat engineers defused 35 mines scattered around the checkpoint, the office said, raising the specter of explosives that could persist long beyond the ongoing conflict. The claim could not be independently verified.

Rebels, meanwhile, said they had taken full control of the main airport in Luhansk, where a military transport plane was shot down last month, killing all 49 soldiers aboard.

Near Kramatorsk, four people died Tuesday and five were wounded when a minibus came under fire, the Donetsk regional administration said in a statement. In the rebel-held city of Slovyansk, heavy shelling destroyed a television tower.

And rebels early Tuesday opened fire on military airplanes at the Donetsk airport, the city’s mayor’s office said. Also in Donetsk, one police officer was killed and six were injured when separatists stormed the regional interior ministry, the Interior Ministry in Kiev said.

Putin stopped short of committing Russia to any specific response to the renewed violence, but he stressed that he would defend any threat to Russians anywhere. He accused the United States of stoking tensions in Ukraine and campaigning in Europe to turn countries against him, calling sanctions against Russia “blackmail.”

“This country will continue to actively defend the rights of Russians, our compatriots abroad, using the entire range of available means — from political and economic to the right to self-defense envisaged by international humanitarian law,” he said.

He said that Russia had been warning for years about NATO’s progressive encroachment toward its borders and that the perception that Ukraine could join the defense alliance was the last straw.

“Everything that Russia has been fighting for since the time of Peter the Great and maybe earlier — the historians will know better — all of that was at risk,” he said.

Stalled negotiations

The Kiev government and self-appointed separatist leaders met for their first talks last week, with former president Leonid Kuchma representing Poroshenko in the negotiations. Poroshenko has offered guarantees on Russian language and increased autonomy to eastern Ukraine as concessions to residents there, but neither side appeared prepared to yield enough ground.

Separatists said Tuesday that they would be willing to return to the negotiating table only if security forces pulled out of eastern Ukraine — a demand that would effectively concede the independence of the regions and one that Poroshenko has ruled out.

The Obama administration and the European Union have been contemplating new sanctions against Russia, although E.U. leaders have appeared reluctant to slap a major trading partner with economic bans that would reverberate at home.

E.U. leaders last week gave Russia a Monday deadline to push rebels toward handing over control of the border to Ukraine and to negotiate with Poroshenko, hinting that new sanctions would soon follow otherwise. E.U. ambassadors met in Brussels on Tuesday but made no announcement about further punitive actions against Russia.

The turmoil in Ukraine was set off in November when Russia-friendly President Viktor Yanukovych abandoned plans to sign an E.U. trade deal, sparking months of protests and his eventual ouster in February. Russia quickly moved to annex Ukraine’s autonomous Crimean Peninsula; U.S. and NATO officials have accused Moscow of stoking separatist sentiments in the east.