By Thomas Grove and Alissa de Carbonnel
MARYNIVKA/LUHANSK Ukraine (Reuters) - Ukrainian border guards stand grim-faced and nervous at the remote Marynivka checkpoint on the frontier with Russia, fearing an attack by pro-Moscow separatists at any time.
Last week they fought off an assault by up to 150 rebels seeking control over supply routes from Russia to bring in arms and other war materials, forcing them to abandon two armored personnel carriers strafed with machinegun fire.
A weary border guard, wearing a camouflage T-shirt and a cap with a Ukrainian national emblem, said he feared the worst if the authorities in Kiev did not send help.
"They told us to expect reinforcements. We're hoping for them soon," said the guard, who gave his name as Vadim. "They (the separatist rebels) drove around us in circles shooting for about four or five hours."
An unexploded rocket-propelled grenade lay in the long grass 200 meters (yards) from the border post.
Not all border guards have put up such a fight. Outgunned and outnumbered, they have fled one post after another in the week since the rebels took the border guards' headquarters in Luhansk, the region's main city.
In an angry letter to the country's defense minister, frustrated Luhansk border guards wrote: "We, including eight among us wounded by bullets and grenades ... sincerely waited for help from you but it never came."
Some of the rebel fighters, who hope to join territory in Russian-speaking east Ukraine with Russia, say they are already able to navigate the border with impunity.
"We need guns, we need supplies from Russia," said a tired-looking rebel, smoking pungent cigarettes in a cafe in the city of Donetsk. He asked not to be identified, fearing punishment if his side loses the conflict.
Ukraine's inability to police parts of its own border underscores the military weaknesses President Petro Poroshenko has to deal with as he tries to end the insurrection that began after his Moscow-leaning predecessor was toppled in February.
His promise to regain Crimea, annexed by Russia in March, also puts him at loggerheads with President Vladimir Putin, complicating dealings with Moscow to plug the power vacuum at the border where Kiev says Russia gives rebels a green light.
"The border can't be closed in a day, and without that the anti-terrorist operation (against the separatists) could continue endlessly," Ukrainian military expert Dmitry Tymchuk wrote on his Facebook page.
REBELS BOAST OF SUPERIORITY
The remaining frontier posts held by Ukraine - built for customs controls, not for war - lie on the outer edge of a swathe of territory crisscrossed by separatists' roadblocks that juts into Russia.
At one backwater border crossing that has fallen to the separatists, at Chervonopartizansk, rebels wave through a steady trickle of cars.
One, dressed in a traditional Cossack fur hat who gave his name as Alexander, said the border guards there had left in a long convoy, taking their weapons and families with them.
"We let them go with their weapons to avoid a fight. Since their position was weaker, we would have had to kill them," he said with a gold-toothed grin.
That may not be an idle threat. Five of the rebels are middle-aged miners but all were ex-military, including veterans of the 1979-1989 Soviet war in Afghanistan.
"I haven't held a gun in 21 years but it's not something you forget," said Vladimir, 41, who said he had also fought in the Balkan wars in the 1990s.
When a column of five vans came bumping down the rutty road toward the border, they jumped into action, quickly reaching for a rocket-propelled grenade and a machinegun.
The vans were filled with women and children fleeing the violence for Russia. The men relaxed, preening before the smiles and shouted thanks from the buses that trundled past.
Further north, at Izvaryne, one of the only border crossings still stamping passports on the Donbass region's 100-km (62-mile) winding border with Russia, a long queue of cars moves slowly along the sun-baked tarmac.
Although government forces are tightening their grip on some rebel strongholds in east Ukraine such as the town of Slaviansk, the separatists appear largely to control the grassy borderlands of Luhansk, the easternmost province.
The only visible signs of a military push are air strikes, but they are not backed up by a coordinated ground presence.
In one incident in Luhansk, which each side blames on the other, eight people were killed when a missile blew a huge hole in the regional administration building occupied by the rebels.
At another separatist base, in a forest outside the city of half a million, markings on shrapnel in the ruins of camp appeared to come from Soviet-design air-to-ground rockets.
The violence is feeding anger against Kiev. Many view the military campaign as a callous and clumsy response to the rebellion against the pro-Western authorities that came to power after President Viktor Yanukovich was overthrown.
"This is a small province. I don't think we can be an independent state - it's not realistic," said Mikhail, 42, a resident of run-down apartment blocks from which rebels attacked the border guards' Luhansk headquarters. "But Kiev's actions are so foul I don't know who will negotiate with them."
Witnesses have reported seeing Russian Kamaz military transport vehicles break through frontier posts into Ukraine.
Even before the rebels took control of any border crossings, some villagers had crossed in and out of Russia using smuggling routes and dirt byways locals call "black roads".
But Moscow and the separatists deny accusations that Russia is allowing weapons and volunteer fighters into Ukraine.
Separatist Valery Bolotov, the self-proclaimed governor of the "Luhansk People's Republic", said his forces were flush with weapons that were pillaged from Ukrainian army and law enforcement bases.
"We are filling our arsenal. We have the means to fight tanks, warplanes and army personnel," he said.
(Additional reporting by Shamil Zhumatov; Writing by Alissa de Carbonnel,; Editing by Timothy Heritage and Will Waterman)