As Ukraine's recently elected president Petro Poroshenko announced today that a Russian invasion of the country had begun, there were no signs of Ukrainian forces on the long stretch of road running along the country's eastern border with Russia, and the new frontline checkpoints in the port city of Mariupol seemed woefully ill equipped to deal with a further advance.

On Thursday afternoon, Poroshenko, who canceled a trip to Turkey to remain in Kiev, said at the country's National Security and Defense Council that "columns of heavy artillery, huge loads of arms, and regular Russian servicemen came to the territory of Ukraine from Russia through the uncontrolled border area."

Kiev and its western allies have long accused Moscow of surreptitiously supporting the uprising in Ukraine's east by allowing men and weapons to flow to the rebels across the two countries' porous shared border. If true, the overt presence of Russian forces on the ground marks a crashing new crescendo in the eastern Ukraine conflict, which according to the latest UN report has claimed at least 1,200 lives between mid-July and mid-August alone.

The allegation comes as pro-Russia forces have quickly seized a cluster of villages and towns stretching out from Ukraine's southeastern border with Russia, including several miles of strategically important highway running from Russia down to Crimea, the southern peninsula annexed by Moscow in March.

Backing up the claims of Kiev, NATO stated that more than 1,000 Russian troops were supporting the insurgent fighters operating in the country's east, and released satellite images to corroborate the allegation.

The rebels' leaders say that up to 4,000 Russian citizens — including veteran and active soldiers — are among their rank-and-file, but claim the fighters are just "volunteers."

"There are active soldiers fighting among us who preferred to spend their vacation not on the beach, but with us, among their brothers, who are fighting for their freedom," Aleksandr Zakharchenko, prime minister of the self-declared Donetsk People's Republic, said in an interview with a pro-Kremlin Russian television channel.

Novoazovsk and all the villages along the nine-mile stretch east of the Russian border are now under the control of the forces of "NovoRosiya," according to Alexey, the commander of the pro-Russia forces holding the area. Alexey spoke to VICE News at a newly established rebel checkpoint just 25 miles from the port city of Mariupol.

Volunteer fighter in Dnipro Battalion on the frontline near Mariupol says his gun was a "gift" from rebels taken after a firefight. Photo by VICE News/Henry Langston.

Men in military fatigues wearing the insignia of the rebel forces refused journalists' entry to the town, where a T-80 tank gun barrel pointed north towards one of the two main artery routes between Mariupol and the Russian border.

This morning Ukrainian forces sealed the other highway running east along the Azov coast. Soldiers cradling automatic weapons and dug in next to two huge stone anti-tank bollards­, painted with flowers and the country's blue and yellow trident, were turning back traffic at the checkpoint — which is separated from the pro-Russian forces positions by only 25 miles of tarmac.

'We have been betrayed by Kiev, where they only care about money,' said one bearded soldier.

Manned by the army, including several conscript soldiers and volunteer fighters from the "patriotic" Dnipro and National Guard battalions, the position is now the first line of defense for Mariupol, a city of nearly half a million. But the men manning the block post on a crossroad near an eastern residential suburb said that without more support from Kiev they would be unable to hold the city for long in the face of an attack.

"We are still waiting for reinforcements, we haven't received anything, no back-up, no weapons. They promised to send help a week ago and we're still waiting," Raff, the unit's sub-commander, told VICE News.

The Dnipro Battalion fighters told VICE News that they were driven out of Novoazovsk and the area to the east after coming under heavy bombardment from mortar and grad rocket fire, with no artillery to fire back.

Craters in the highway near the village of Bezimenne which is now on the frontline of fighting in east Ukraine. Photo by VICE News/Henry Langston.

"We have been betrayed by Kiev, where they only care about money," said one bearded soldier, who pointed anxiously down the road running to the east, where he claimed 30 Russian tanks were stationed. "We know they are there because we saw them with our own eyes as we retreated from the town."

"They'll hit the petrol stations, and probably those apartment blocks over there. The city's mayor should start evacuating people," said Raff, gesturing to the buildings surrounding the Ukrainian position. "Another attack is overdue, we expected it yesterday."

Less than 18 miles down the road, near Bezimenne, a village now awkwardly positioned on the front line, the anticipated assault seemed to have tentatively begun, as a first round of grad rocket fired from the direction of Novoazovsk fell just shy of another Ukrainian position at around 5pm, smashing craters into the tarmac. The soldiers stationed there told VICE News that they had seen rebel spotters operating in the area earlier in the day, and expected further assaults on the area.

Meanwhile, Mariupol center was a sea of blue-and-yellow flags, as up to three thousand people gathered for an emotional peace rally as war knocked at the city's door.

The next few days will prove "decisive," a security source in Kiev told VICE News.

"Mariupol is the big test," he said, warning that the pro-Russia front could be pushed all the way down to Ukraine's recently annexed southern peninsula, if Moscow was pleased with latest action's results. "If this campaign is successful they will move troops out of Crimea and join the two forces up."

Follow Harriet Salem on Twitter: @HarrietSalem