The boom and whoosh of outgoing howitzer and Grad rocket fire echoed through streets of central Donetsk this weekend as pro-Russia forces appeared to step up an offensive on the airport where they have been battling the Ukrainian army and volunteer battalions for more than five months.

Thousands of people have returned to the city in the last month after fleeing a campaign of heavy shelling by Ukrainian forces over the summer. An increase in the movement of military equipment and fighters around the rebel-held territories, however, has prompted concern from Kiev and its western allies that fighting is about to go up a notch again as pro-Russia separatists begin a campaign to seize new territory.

On Sunday alone, the Organization for Security and Cooperation in Europe, who maintain a monitoring mission in Ukraine's east, reported seeing two large convoys of military vehicles including four T72 and five T64 tanks, 19 howitzers and more than 40 Kamaz trucks and tankers without license plates.

Several international journalists have also spotted processions of military equipment moving around the region over the last week.

The OSCE monitors also observed "personel in dark green uniforms without insignia" a description that matches the soldiers deployed from Russia during the covert invasion of Crimea earlier this year.

On Friday, authorities in Kiev said they had evidence of a fresh convoy of military equipment including "32 tanks, 16 D30 howitzer and 30 Kamaz trucks loaded with arms and fighting forces" entering from Russia into Ukraine's east via Krasny Luch.

According Colnel Andriy Lysenko, a spokesperson for the Ukrainian army, pro-Russian forces have intensified their attacks on army positions. "The past week was characterized by an increase in the intensity of shelling and the transfer of additional force — ammunition, equipment and personnel — terrorist groups" he said in a statement on Sunday.

In response to the mounting tensions, Ukraine's president Petro Poroshenko has ordered reinforcements of troops to be sent to frontline positions in Kharkiv, north Luhansk and Mariupol.

However, rebel leaders have denied launching a new offensive and receiving more supplies of weapons from Russia, claiming that the spotted convoys just reflect the "rotation of fighters."

"The front line here is very big, and parts of it are in various different regions," Eduard Basurin, deputy commander of the Donetsk People's Republic militia told Russian state news agency RIA Novosti. "The column arrived from one of those."

NATO has not confirmed an increase in the presence of Russian military forces or the transfer of new equipment from Russia into the rebel-held territories, but has long noted that the border is "porous" and "open".

In a statement released on Sunday, EU foreign policy chief, Federica Mogherinis called the situation "very worrying" and called on the Russian Federation to "prevent any further movement of military weapons or fighters from its territory into Ukraine and withdraw any troops, weapons and equipment under its control from Ukraine."

For now, however, the fighting still appears to be contained to flashpoint spots and the city center is not under the heavy bombardment of the summer months.

But as Ukraine's bitterly cold winter approaches, the humanitarian situation in areas near the frontlines — where fighting continues on a near daily basis — is dire.

"What can I tell you? Half of the district is destroyed. There's no central heating, the electricity cuts in and out, there's only cold water… a quiet day means just a couple of shells landing in the neighborhood," Alexander, a 35-year-old builder who lives with his family in the Kuibyshevskiy district next to the airport, told VICE News. "My windows were blown out three days ago by mortars… around 20 percent of people from my [apartment] block were still living here, but after the latest shelling more people left."

Two teenage boys were killed by a mortar shell which landed in a school sports field where they were playing football last week. In some of the residential neighborhoods near the frontline, hundreds of residents have been living below the ground in Soviet-era nuclear bomb shelters for several months.

Both the warring sides continue to blame each other for violating a ceasefire deal which was agreed in September, but which in practice has long only existed in name. According to the latest figures from the United Nations, at least 4,000 people have been killed in the conflict in Ukraine's east since fighting broke out in mid-April, with more than 400 deaths recorded after the ceasefire came into effect.