ROSTOV-ON-DON, Russia - It was at a barbecue outside a city in southern Russia, this Russian war veteran recalled, that he and some friends met an envoy for the separatist rebels in eastern Ukraine. They were fighting for their families, the envoy said, against the "neo-Nazis and ultranationalists" who had seized power in Kiev in a February coup.
"We asked him, 'Do you need help?' " the veteran said in an interview on the Ukrainian side of the border. "He said, 'Yes we do.' And so we offered to help."
As warfare has raged in the separatist provinces of Donetsk and Luhansk, Russia's role has been hard to discern. In contrast to the conflict in Crimea, no regular Russian soldiers have been visible in eastern Ukraine, and President Vladimir V. Putin has ordered his troops to pull back from the border region.
But late last week, Ukrainian border guards abandoned a post near Luhansk, blowing a hole in a border that was already considered porous at best, opening the way for men, war matériel and contraband to enter the country. Since then, several other posts have been commandeered by the separatists, who also overran the border guards' headquarters in Luhansk.
The overall situation at the frontier remains murky; there is no evidence yet of a flood of men and arms crossing the border. Yet it is clear that numerous Russians, most of them war veterans, Cossacks or ultranationalists, have signed up to fight in Ukraine in recent weeks, either with recruiters or through one of several websites established expressly for enrolling them.
Exact figures are elusive, but analysts say that a significant number of Russian citizens have slipped across the border in recent weeks. In one indication, after a particularly intense battle last month, a leader of the self-proclaimed Donetsk People's Republic said 31 of the 40 or so fighters killed were Russians, their bodies subsequently returned for burial in their homeland.
Moscow, held responsible by the West for nurturing the conflict, denies any role in the war, yet it clearly has done almost nothing to stop its citizens from joining the fight. On Saturday, in response to calls from the Group of 7 countries, Mr. Putin ordered the Federal Security Service to tighten the border to stop illegal crossings. But the effects of that order were not immediately evident.
The Dolzhansky checkpoint, one of several on the Russian border that have been taken recently by pro-Russian separatists, was manned last weekend by Cossacks, the traditionally semi-nomadic Russian horsemen of the southern steppes who live in villages on both sides of the border - sometimes even straddling the boundary.
There was no sign of the men, arms and ammunition that the Ukrainian government says are crossing the border in increasing numbers from Russia and stoking the conflict in eastern Ukraine. Sitting on lawn chairs in the shade, the Cossacks waved through a trickle of dusty cars. Some carried men, others women and children. They were not bothering with a passport stamp; they said they did not even have one.
Anybody could pass into Ukraine, one of the fighters said, but guards on the Russian side were not letting weapons through because that would be "an act of war."
Carol Saivets, a Russian specialist for the Security Studies Program at the Massachusetts Institute of Technology, said the switch from reliance on local eastern Ukrainian men to a force with more Russians started last month. It is almost certainly proceeding with the blessing and backing of the Kremlin, she said, "even if the Russians are indeed volunteers rather than serving military men."
She added, "Putin has so stirred up Russian nationalism, and with the propaganda barrage that there are fascists in Kiev, it's not inconceivable that some lost souls will sign up to serve.
"It's also clear that Russia has been assisting them in their efforts," she said, "and now, as it becomes an increasingly professionalized force, there have to be questions about how much support Russia is giving. My bet is a lot."
Not everything, certainly not everyone, coming into Ukraine has to cross at a checkpoint. Outside the official crossing areas, the borderlands are a porous landscape of wheat fields, oak forests and innumerable ravines and country roads, crisscrossed by militants who can easily escape detection.
The route to the frontier from the Russian side leads across the Don Steppe, an area known in czarist times as the "Wild Field." It is a rural and agricultural panorama, where ripe winter wheat blows in the breezes, interspersed with dense forests and tiny villages.
On the Russian side of the border stand new concrete buildings and proper fencing, separated by 100 yards or so of potholed asphalt from the Ukrainian side, with its sheet metal guard shacks. Afterward the road carries on, through the same countryside.
Most men come across unarmed, picking up rifles later on that might have been pilfered from Ukrainian military arsenals, web testimonials and Russian volunteers interviewed say.
The soldier recruited at the barbecue, who offered only his first name, Alan, for security reasons, said he slipped into Ukraine in May, long before Mr. Putin's announcement. The recruiter was a pro-Russian separatist from Ukraine, he said in an interview at a separatist base in Donetsk. "I don't take commands from the Russian government," he said.
While the personal touch is no doubt effective - often reinforcing the idea hammered at in the Russian news media of battling neo-fascists who seized the Ukrainian government in a February coup - volunteers wanting to join the battle can simply click on any of several recruitment websites.
"We help people who wish to provide military charity to brother nations and states," says one site providing the service, dobrovolec.org . (Dobrovolec means "volunteer.") "In other words, we help you to help others."
A facilitator at the site, Aleksandr Zhuchkovsky, a 27-year-old native of St. Petersburg, said in a telephone interview that he helped a dozen or so Russian nationalists cross the border last month, and has as many lined up for June.
"In my experience, the government doesn't help, but doesn't prevent us from getting through," he said. Volunteers live in hotels, rented apartments and tent camps in Rostov-on-Don, a staging area for the activity.
Interviewed on the Ukrainian side of the border, one volunteer in the Vostok Battalion, Aleksandr, 18, said he learned of the volunteer effort while surfing Yandex, the Russian version of Google, then joined through an acquaintance. Volunteers could come, he said, but "Putin cannot send the army, because that would set off World War III."
Cossacks, are also volunteering, though they complain about being restrained by Moscow.
"We don't need a website to cross the border," Ivan P. Doroshev, a commander of a Cossack unit, the 10th Georgiyevsky Hundred of the Don Cossacks, said in an interview, over a meal of salted fish at his country retreat in the reedy wilderness of the Don River delta, on the Russian side.
It is easy enough to walk across the border, he said. But the Russian government has prohibited Cossack units from crossing en masse. Instead, individual volunteers go to fight. Four from his village have gone, Mr. Doroshev said. One was killed and three returned wounded.
"If they volunteer, they volunteer as individuals and not as my men," Mr. Doroshev said. Still, among eastern Ukrainian militants, Cossack symbols like sheepskin hats, or the Don Cossack symbol of a deer struck by an arrow - meaning it is better to die free than to live like a slave - are commonplace.
A Facebook eulogy to a Russian nationalist who died in Ukraine, Aleksandr Vlasov, cited the young man's posts before leaving, and his plans. "We should receive assault rifles" on the Ukrainian side, he said. He planned to buy a uniform and a rucksack in Russia, then slip across.
"Sure, I don't want to die and leave my children and wife and living mother," he wrote. "But it would be worse to someday have my son ask, 'And what did you do while the Nazis killed people?' " He finished by writing, "Russians forgot how to die. And we die marvelously. Like none others."
Andrew E. Kramer, NYTimes.com