Ukrainian security forces on Tuesday pressed an assault against pro-Russian separatists in the east of the country, hours after Ukrainian President Petro Poroshenko suspended a 10-day cease-fire that had brought only limited tranquility and no permanent peace.

Both sides appeared to be digging in for a protracted conflict, as separatists said Tuesday that they would be willing to return to the negotiating table only if Ukrainian security forces pulled out of eastern Ukraine — a demand that Poroshenko has flatly ruled out.

Four people died Tuesday and five were wounded when a minibus came under fire near the eastern town of Kramatorsk, the Donetsk Regional Administration said in a statement. It said that an investigation was still under way and did not make clear whether rebels or Ukrainian security forces did the shooting. Rebels early Tuesday opened fire on Ukrainian military airplanes at the Donetsk airport, the Donetsk mayor’s office said.

Poroshenko on Tuesday said he would restart military operations against separatists in the east of the country, vowing to strike a blow against rebels after a 10-day cease-fire ended with no peace deal.

Declaring in a nationally televised speech that pro-Russian separatists who have seized territory in the east had met none of his demands for peace talks, Poroshenko said that he would be ready to return to the negotiating table if his opponents released hostages, allowed international monitors on the borders and halted weapons flowing in from Russian territory.

“We will attack and defend our land,” Poroshenko said in the speech, which came after an hours-long meeting with his advisers. Protesters had gathered outside his offices in Kiev to push for an end to the cease-fire, which many on the pro-government side had worried was simply giving the separatists a chance to regroup and rearm. In the speech, Poroshenko guaranteed Russian-language rights and more regional autonomy, two key demands of Kiev skeptics in the country’s rebellious industrial heartland in the east.

Russian leaders on Tuesday condemned the truce’s end and called for a return to peace talks.

“We think that it is simply impossible to restore peace, justice, law and order in Ukraine without a truce and without starting dialogue,” the chairman of the lower house of Russia’s parliament, Sergei Naryshkin, said on Tuesday, according to the Interfax news service.

Violence had continued during the 10 days of the cease-fire, but its intensity had lessened, Ukrainian officials had said in recent days. The official end to the break in hostilities appeared to put an end to halting attempts to bring the sides to the negotiating table and it raised the prospect of strengthened sanctions by the European Union and the United States against Russia.

E.U. leaders on Friday hinted that they would slap Russia with more sanctions if it did not take concrete steps to seal its porous border with Ukraine and ensure that separatists put down their arms. U.S. officials also have indicated that they are poised to increase measures against Russia. White House spokesman Josh Earnest said Monday that the Kremlin needed to take “tangible actions” to avoid more sanctions.

Poroshenko spoke on Sunday and Monday to Russian President Vladimir Putin, German Chancellor Angela Merkel and French President François Hollande in a last-ditch effort to prolong the cease-fire, Hollande’s office said.

Ukraine’s Foreign Ministry said that 27 troops had been killed after Poroshenko announced the cease-fire June 20.

Russian Foreign Minister Sergei Lavrov on Monday called for a “full-fledged peace process” as soon as possible, the Foreign Ministry said. Russia is willing to post Ukrainian and international observers in its own border posts along the border with Ukraine, Lavrov said, an apparent effort to satisfy a European demand that separatists hand back Ukrainian border points that they have seized in recent weeks.

Pro-Russian separatists have said they would be willing to engage in full negotiations only if Ukrainian troops make a full withdrawal from territory that they have claimed as sovereign and independent — an unlikely step, since Ukrainian authorities have said that they worry that doing so would simply allow their opponents to rearm.

Meanwhile, a top American NATO official renewed accusations Monday that Russia has been supporting the separatists with weapons and other support, a charge Russian officials have denied.

“There are some good words about a cease-fire and peace, but what we see is continued conflict, continued support of the conflict from the east side of the border” with Russia, the commander of NATO forces in Europe, Air Force Gen. Philip Breedlove, told reporters in Washington.

Also Monday, Russia’s state-run Channel One announced that a cameraman, Anatoly Klyan, 68, died near the eastern city of Donetsk after a bus he was traveling on came under fire, apparently from the Ukrainian military.