Residents of the eastern Ukrainian city of Donetsk are wary of the cease-fire between government forces and pro-Russian separatists, but are hoping for the best. (Reuters)

Residents of the eastern Ukrainian city of Donetsk are wary of the cease-fire between government forces and pro-Russian separatists, but are hoping for the best. (Reuters)

Ukraine’s fragile cease-fire appeared to be in danger late Saturday, slightly more than 24 hours after it took hold, as witnesses reported renewed fighting near the key government-held city of Mariupol.

Arsen Avakov, Ukraine’s interior minister, said on his Facebook page late Saturday that there had been “violations of the cease-fire from Russia,” with Grad rockets fired 16 times at Ukrainian positions “from Russian territory.” He said Ukrainian forces in Mariupol were being reinforced with additional troops, including the First Brigade of the National Guard.

Mariupol, 25 miles west of the Russian border, is at the maximum of the multiple rocket launcher’s typical range, and it was not clear whether Avakov was referring to an assault on the city.

There was no immediate response from Russia, but the Kremlin has in the past denied Ukrainian accusations that it is firing on Ukraine from its territory.

“Are you surprised that Putin is treacherous?” Avakov wrote, referring to Russian President Vladimir Putin. “This has not canceled our determination to defend Mariupol.”

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The bombardment and explosions to the east of the industrial city on the Sea of Azov came at the end of a mostly calm day. It was not clear whether the fighting near Mariupol represented a full-fledged end to the cease-fire or simply a localized burst of violence, and after about two hours the explosions had quieted, witnesses said.

The fighting underlined the speed with which the Ukraine tinderbox could reignite after nearly five months of hostilities. Ukrainian government forces and the pro-Russian rebels who have seized territory in the east have said that they are weary of combat — but neither side was confident Saturday that the cease-fire would hold for long.

Witnesses in Mariupol said that the easternmost government-held checkpoint on the outskirts of the city of 500,000 came under fire around 10:30 p.m. Saturday. Ambulances raced to the fighting, and Ukrainian military vehicles sped through the city.

“I was at a café by the seaside with a friend when I heard the shelling start,” said Vladimir, a 29-year-old Mariupol factory worker who asked that his last name not be published because he feared for his safety. “We heard the sounds of the waves and then explosions.”

After 35 minutes of bombardment apparently directed at government positions, he said, he heard what sounded like the Ukrainian military firing back, although it was difficult to determine the exact situation at the late hour.

On Friday, envoys from Kiev and the pro-Russian rebel group signed an agreement to lay down their arms temporarily in the hope of finding a durable peace. Putin’s and Ukrainian President Petro Poroshenko’s telephone diplomacy last week helped hammer out the temporary truce, and earlier Saturday they had agreed in another telephone conversation that the cease-fire was “generally” holding up, according to statements released by each leader. They also discussed facilitating the delivery of humanitarian assistance to the war-torn regions of eastern Ukraine — the next step of the cease-fire along with a prisoner swap.

But there was no mention by either president of the long-term future of the rebel-held territories, a reminder of the tenuousness of the peace. At a NATO summit in Wales, where the Russia-Ukraine conflict was at the top of the agenda, Poroshenko on Friday offered more political and cultural autonomy for eastern Ukraine. His offer, though it was short on specifics, was similar to one made in June that was rejected by the rebels.

View Graphic This map shows the latest available front lines in Ukraine.

The Kremlin and the rebels have pushed for, at a minimum, vastly more political and economic independence for eastern Ukraine. Because of the region’s historic cultural and business ties to Russia, that would give the Kremlin far more say in Ukraine’s national destiny than many in Kiev would be comfortable with.

Some rebel leaders Friday said they would be satisfied with nothing less than full independence. That, Poroshenko said Saturday, would be a non-starter.

“I am ready to fight for my country, and I am sure a huge number of Ukrainian residents think the same. The territorial integrity, sovereignty, and independence of Ukraine will remain as they are,” Poroshenko wrote on Facebook.

Parliamentary elections scheduled for Oct. 26 make concessions particularly difficult for Poroshenko, since any deal that would in effect freeze the conflict would be unpopular among his supporters.

Some Ukrainian citizens fear that the country’s leadership decided to agree to a truce because its forces simply could not withstand the recent offensive in the region backed up by what NATO says are trained Russian troops.

“The Ukrainian government has to explain to its own nation exactly what is happening, and it hasn’t done so yet,” said Konstantin Batozsky, an adviser to Serhiy Taruta, the Kiev-appointed governor of Donetsk. He said many residents were highly skeptical that the cease-fire would endure.

“The people of Ukraine will not accept peace at any price,” he said.

In a measure of the low international expectations about the durability of the peace, the European Union readied new sanctions against Russia that it was due to approve Monday. The fresh measures would target the energy, finance and defense industries, along with more individuals. E.U. leaders said the sanctions could be rolled back if the truce holds.

Those plans met Saturday with a sharp rejoinder from the Russian Foreign Ministry, which said in a statement that the E.U. is “sending a signal of direct support to the party of war in Kiev.”

The International Red Cross, in a tweet, said that shelling Saturday in rebel-controlled territory forced an aid convoy destined for Luhansk to turn back. The organization did not say which side was doing the firing.

Faiola reported from Kiev, Ukraine. Alex Ryabchyn and Natalie Gryvnyak in Kiev contributed to this report.