The battle for Ukraine's strategic coastline has intensified as a local mayor reported that pro-Russian rebel forces entered a key town in southeast Ukraine after three days of heavy shelling.

Novoazovsk, a resort town of 40,000 on the Sea of Azov, lies in a strategically significant location, on the road linking Russia to the Ukrainian port of Mariupol and onto Crimea, the Black Sea peninsula Russia annexed.

Wednesday's incursion was the first time in the four-month-long conflict between the government in Kiev and separatists in the east that fighting has reached as far south as the seacoast.

The new southeastern front has raised fears the separatists are seeking to create a land link between Russia and Crimea.

If successful, it could give them or Russia control over the entire Sea of Azov and the gas and mineral riches that energy experts believe it contains.

Ukraine already lost roughly half its coastline, several major ports and significant Black Sea mineral rights in March when Russia annexed Crimea.

Oleg Sidorkin, the mayor of Novoazovsk, told the Associated Press by telephone that the rebels had entered the town and he had seen dozens of tanks and armoured vehicles roll in.

Sidorkin said the rebels had been positioned near Ukraine's southernmost border with Russia.

The assault on the town has forced government troops to spread their ranks thinner along the Russian border.

A spokesman for Ukraine's security council, Colonel Andriy Lysenko, said "we do not have information that it [Nozoazovsk] is occupied."

In the town of Starobesheve, about 30 kilometres southeast of Donetsk, there were signs of a hasty departure by Ukraine's army.

Locals told the AFP news agency that the troops left on Monday after shelling started from the direction of the Russian border about 30 kilometres away.

Ukraine's military conceded that "militants together with Russian occupants" had taken control of Starobesheve, as well as a string of villages near Novoazovsk, a town on the Azov Sea where clashes had been raging for days.

In Mariupol, a city of 450,000 about 30km to the west, the defences built up.

A brigade of Ukrainian forces arrived at the airport on Wednesday afternoon, while deep trenches were dug a day earlier on the city's edge.

In Donetsk at least three people were killed on a main road when their cars were hit by shrapnel from falling artillery shells.

'Direct intervention'

There is increasing concern in Kiev over Russia's willingness to intervene directly in the conflict with the Ukraine government claiming on Wednesday that a battalion of Russian soldiers had set up a military headquarters near the village of Pobeda, around 50km southeast of Donetsk.

NATO and Polish intelligence also have evidence of regular Russian army units operating in Ukraine, Poland's Prime Minister Donald Tusk told his parliament on Wednesday.

Russian Foreign Minister Sergey Lavrov responded that the Kremlin was "not interested in breaking up" Ukraine, although it does respect two independence referendums from May - in Crimea and the Russian-speaking eastern regions of Luhansk and Donetsk.

Lavrov also confirmed that Russia will send a second aid convoy into Ukraine "in the nearest future" which will be followed by others.

A first convoy was sent to Luhansk last week, and Kiev fears the convoys could provide cover for smuggling military support to the rebels.