Stranded on vacation on Mexico's Caribbean coast when Russia invaded Ukraine, Gorpenko, 47, has now spent more than three weeks without hearing from the 72-year-old Vladimir Gorpenko.

He does know that bombs and shelling have destroyed much of Mariupol, once a city of 400,000 people, thousands of whom are trying to escape. Vladimir's home was not spared; a photo circulating online shows the multi-story building burned out.

To find his father, Gorpenko posted a photo on Facebook showing a smiling man with thinning white hair. He is trying to get friends in Ukraine to rescue him.

"It's been impossible because Mariupol is under constant bombardment," Gorpenko told Reuters from the Mexican beach getaway of Playa del Carmen.

Reuters was unable to independently verify his account.

The hotel extended his stay at a tenth of the original price after he explained he could not go home, and has given him internet access to look for clues of his father.

"The first five days we were in touch ... I hope Dad is still alive and hiding in a bomb shelter."

'OUT OF THIS HELL'

As Russia keeps up air strikes to take the strategic port city that could help it link areas held by pro-Russian separatists in Crimea, Ukraine says 100,000 people in Mariupol are trapped - cut off from food, water, power and heat and blocked by fighting from being able to escape.

The Kremlin denies targeting civilians and says its forces are engaged in a "special operation" to demilitarize and "denazify" its neighbor. The West and Kyiv call that a false pretext to invade a democracy.

Russia and Ukraine have close historical ties, with family spread out across both nations.

Gorpenko's 21-year-old son, a Russian citizen, is finishing a university degree in Moscow, and Gorpenko fears he could be drafted into Russia's army.

Dozens of Ukrainians have fled to Mexico in recent days with hopes of crossing into the United States, and many have been allowed by U.S. officials to enter the country and stay without fear of deportation for a year.

Gorpenko was already in Mexico when the Russian invasion began on Feb. 24, having left Ukraine a week earlier for the beach vacation with seven friends. His companions, with children in Ukraine, went back to Europe.

With many friends still in Mariupol, Gorpenko knows he is not the only one trying to bring people to safety.

"Everyone is now searching, trying to somehow save their loved ones and get them out of this hell," he said.

More than 4,000 people fled Mariupol on Saturday. Ukraine and Russia on Sunday agreed to a humanitarian corridor for private cars to leave the city.

Once he knows his father is safe, Gorpenko plans to go to the United States.

"I can't afford to go back to Europe," he said. "Besides, there's nothing left of my Mariupol and nothing to go back to in Ukraine."

(Reporting by Dasha Afanasieva in London, Daina Beth Solomon in Mexico City and Paola Chiomante in Playa del Carmen; Editing by Dave Graham and Daniel Wallis)

By Paola Chiomante, Dasha Afanasieva and Daina Beth Solomon