'Mariupol is Ukraine'

Compared to other cities affected by conflict in eastern Ukraine, the center of Mariupol is bustling. Shops are open, the streets full of cars and buses. People are going to work.

But echoing across the main Lenina Avenue is the sound of shelling. Just east of the city Ukrainian government forces are exchanging artillery fire with rebel troops. The rebel leadership in Donetsk has repeatedly threatened to retake Mariupol, which its forces previously held before being driven out by Ukrainian troops.

The newly elected rebel leader, Alexander Zakharchenko, says the Minsk ceasefire agreement is ambiguous in defining where separatist-held territory ends. He believes Mariupol belongs to the self-proclaimed Donetsk People's Republic.

The rebels, increasingly isolated in a pocket of land next to the Russian border with limited supplies, desperately need the southern port city to trade goods.

'Mariupol is Ukraine. We are Ukraine'

And so tensions here are rising. A Ukrainian checkpoint on the road leading to the Russian border is now completely closed, the road ahead towards the frontline and the rebel-held town of Novoazovsk eerily silent.

We try to accompany a convoy of observers from the Organization for Security and Cooperation in Europe( OSCE) on a trip east to the Russian border, but without special permission only the official vehicles get through. Even a local man on his bicycle is turned away.

The Ukrainian soldiers are nervous, fearing the next rebel attack. And within hours, it comes. "We've recorded an increased number of incidents to the east of the town," says Alexander Hug, the OSCE's deputy head of mission, speaking on a visit to Mariupol.

But local residents are stoic, and many remain loyal to Kyiv. "Mariupol is Ukraine. We are Ukraine," one old lady tells me.

At another frontline checkpoint on the road between Mariupol and Donetsk, Ukrainian soldiers from a Kyiv-based battalion carry out final controls on traffic driving in and out of rebel territory. They say fighting has escalated in recent days and they are now expecting a major assault from forces who they describe as "Russians."

"We have very heavy artillery fire from the Russian side on the Ukrainian villages, border line and Ukrainian forces, so now we are preparing for their attack," says Yegine Karas, a soldier at the checkpoint. "But when they come here they will take very big casualties because it's our land and we will not retreat."

Looking for reinforcements

The checkpoint is isolated and exposed, particularly as night falls. A lookout soldier stands on a roof, scanning the surrounding fields through binoculars. The troops at this checkpoint also act as eyes and ears for fighting units dispersed in the countryside near rebel positions and back them up from sniper positions.

Ukrainian President Petro Poroshenko has pledged to send more troops to bolster forces in the east. At this particular checkpoint, that hasn't happened. "We don't know when we will have reinforcements here," says Karas. "But I think the troops we have here are greatly motivated and also really ready to defend this land."

But there have been multiple unverified reports of rebels also bringing more military hardware and reinforcements into the region, possibly from Russia. The conflict looks set to escalate further.