— Soldiers fortified trenches and protesters formed a human chain Saturday to try to defend this strategic port city in southeastern Ukraine as fear spread that Russia would expand its incursion into its neighbor.

Military analysts think Mariupol could be a next target because it has access to the sea and also would provide a valuable land bridge to Crimea, the former autonomous Ukrainian territory annexed by Russia in March. Rebels supported by Russian soldiers, tanks and armored personnel vehicles seized control of the town of Novoazovsk — just 30 miles east of here — Thursday, according to Ukrainian military officials.

On Sunday morning, residents said the town had quieted, although it remains under insurgent control.

The West has grown increasingly alarmed by what it considers Russia’s brazen push into Ukraine. On Saturday, European Commission President José Manuel Barroso said the crisis in eastern Ukraine could soon near “a point of no return” and that Europe might impose new sanctions on Russia. Ukraine’s president, Petro Poroshenko, warned that the conflict could spread further into Europe. Russia denies it is sending military forces into Ukraine.

On Capitol Hill, Republican Sens. John McCain (Ariz.) and Lindsey O. Graham (S.C.) urged the administration to immediately supply weapons to Ukraine and to increase economic sanctions on Russia.

The capture of Novoazovsk could open a new front in the five-month-long Ukraine conflict, which until now has centered on the Donetsk and Luhansk regions, farther to the north. The small border town appears to be firmly in the hands of Russian-backed separatists, who flew flags of a new territory they are calling Novorossiya . A spokesman for the rebels told the Associated Press on Friday that their plan was to push onward to Mariupol.

Fighting continued Saturday in other parts of eastern Ukraine. In Ilovaysk, a southeastern town that has been besieged by pro-Russian separatists, 28 of more than 200 Ukrainian soldiers trapped in the town managed to escape.

The Ukrainians returned 10 Russian paratroopers captured on its territory to Russia, and Russia handed over 63 Ukrainian soldiers who had crossed the border, a Russian military official in charge of the airborne forces told Russian media.

Ukraine trumpeted the capture of the paratroopers last Monday as proof that Russians were operating a “special mission” in Ukraine, while Russia claimed that they had only crossed by accident.

Although Mariupol was quiet Saturday, a Ukrainian military spokesman in Kiev showed reporters leaflets he said were being handed out in Novoazovsk offering money for information on Ukrainian troop movements and instructing locals on how to prepare for the arrival of “peacekeeping troops of the Russian Federation.” There was no confirmation of who had printed up the leaflets.

Ukrainian military spokesman Andriy Lysenko said that the army was ready to defend Mariupol, having organized round-the-clock patrols and reinforced entrances to the city. Hundreds of Ukrainian army troops were at posts around the city, according to Mariupol’s mayor, Yuriy Khotlubey.

Mariupol’s residents were preparing in their own way. Many had stocked up on bread and other provisions. There were long lines of cars exiting the city through checkpoints. Supplies of some medicines ran low. More than 800 basements and shelters had been designated for use in case of shelling, the mayor said.

The city gave free train tickets to refugees from other parts of the war-torn country so they could flee yet again — to safer areas.

On Saturday, protesters held hands and chanted “Out with Putin!” at a checkpoint on the eastern edge of Mariupol.

Other citizens chose to preserve their routines. Residents strolled in parks with their dogs and children on a cool end-of-summer day. Outdoor concerts, picnics and two weddings – the mayor noted – went on as scheduled. Occasional convoys of cars careened down the streets, their occupants honking and waving blue-and-yellow Ukrainian flags.

But the threat just up the road loomed large.

“We are living in this town as peaceful citizens, but we know the tanks are coming,” said Vladimir Marchenko, a sailor. “We don’t want to become part of another country.”

The residents, a mix of ethnic Russians, Ukrainians and Greeks, have endured considerable turmoil this year. After the country descended into civil conflict following mass protests against the country’s president earlier this year, pro-Russian separatists established a strong presence in the city from April 13 to May 9. Their tenure was capped by a firefight that killed nine at the police headquarters.

NATO officials and Ukraine’s government say that Russia has been sending military equipment and hundreds of soldiers into Ukraine to help the separatists. Russian officials have denied the charge — saying some Russians volunteered to assist the rebels, and some wandered into Ukraine by mistake.

Some residents of Mariupol say they would welcome Russian soldiers if their tanks rolled into Mariupol.

“If the Russians would come here there would be no war. No one would be killed. It would be like Crimea,” said Natalia Obolonskaya, a nurse. “I would feel better with the Russian army than the Ukrainians.”

Others who remembered life before Ukraine became independent in 1991, in the waning days of the Soviet Union, feel differently.

Ludmila Elagina, a retired engineer, volunteered to help dig trenches in the city this past week because she fears the return of a repressive regime.

“When we lived under the U.S.S.R., we felt we were being controlled,” she said. “We were told what to wear. We were told what to say. Independence was like a second life, the birth of something new. My wings spread and I started to remember poems I recited to my mother as a child. Now, I have poems of terror.”

The pressures of living in a politically divided city have taken their toll, and the anxiety has worsened in the last week, according to Ulyana Tokareva, the director of the city’s social services center.

“Their two main questions are, ‘What is happening, and what do I do next?’ People are panicking right now,” she said. “The panic is happening because people don’t know who to trust.”

At a modest brick building Saturday, groups of families who had fled the fighting in the rebel-controlled cities of Donetsk and Luhansk arrived, toting their possessions, to pick up free train tickets out of the city.

Ludmila Kosych, 55, who ran a small food store, fled with her family from the Donetsk region Aug. 20 after they witnessed continued horror: missile fire, dead children, an average of 10 funerals a day.

They thought they had reached a safe place in Mariupol. But now she said she was wrong.

“Imagine if a bomb is flying over there,” she said, gesturing to a nearby column of trees. “It explodes and by any piece of it you could be killed. You don’t know where it’s coming from, the Ukrainians or the rebels. It’s human, animal fear.”

Karoun Demirjian contributed to this report from Moscow. Natalie Gryvnyak contributed from Mariupol.