MARIUPOL, Ukraine (AP) — The battle for Ukraine’s strategic coastline heated up Wednesday 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, reported by the town’s mayor, 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. It suggests that the rebels — who Ukraine, NATO and Western nations all say are being supported by Russia — have been both emboldened and reinforced.

The new southeastern front also 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 armored vehicles roll in.

AP reporters earlier in the day saw more than 20 shells fall around the town in a one-hour span. But access from the west was blocked by Ukrainian soldiers later and the presence of rebels in Novoazovsk could not be independently confirmed.

Sidorkin said the rebels had been positioned near Ukraine’s southernmost border with Russia. It was not immediately clear how the rebels could have traveled to the southeast area, which is far from the main front line further north and in an area controlled by the government. Fighters could have easily come over the Russian border, however.

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, Col. Andriy Lysenko, said “we do not have information that it (Nozoazovsk) is occupied.”

Earlier, he said the shelling around the town was coming from both Ukrainian and Russian territory. Ukrainian security officials said nearby villages had also come under shelling.

In Mariupol, a city of 450,000 about 30 kilometers (20 miles) to the west, the defenses 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. Other troops were blocking traffic from entering or leaving the port’s eastern edge.

Artillery shells in Novoazovsk appeared to be flying between rebel and government positions.

“It hit a tree, there was a blast and the shrapnel came down here,” said Alexei Podlepentsov, an electrician at the Novoazovsk hospital, which was struck by shelling Tuesday.

In Donetsk, the largest rebel-held city further north, at least three people were killed on a main road when their cars were hit by shrapnel from falling artillery shells.

Fighting persisted elsewhere, and Lysenko said 13 Ukrainian troops had been killed over the past day.

Ukraine and Western governments have repeatedly accused Russia of playing a direct role in the conflict, supplying troops and weaponry to the rebels. Russia denies the claims, but their stance is increasingly dismissed abroad.

“Information, which in recent hours has gained another hard-facts confirmation, is that regular Russian units are operating in eastern Ukraine,” Polish Prime Minister Donald Tusk said Wednesday. “This information, coming from NATO and confirmed by our intelligence, is in fact unequivocal.”

Russian President Vladimir Putin and his Ukrainian counterpart, Petro Poroshenko, met in the Belarusian capital of Minsk on Tuesday for their first ever one-on-one meeting, which lasted over two hours. But there was no indication of a swift resolution to the fighting that has dragged on since April and claimed at least 2,000 civilian lives.

Poroshenko called the talks “overall positive” and said Putin had accepted the principles of his peace plan, which includes an amnesty for those in the east not accused of serious crimes and calls for some decentralization of power.

Putin, however, insisted that only Kiev could secure a cease-fire deal with the pro-Moscow separatists, saying the conflict was “Ukraine’s business” because Russia was not in the fight.

Russia “can only help to create an atmosphere of trust for this important and necessary process,” Putin said. “We in Russia cannot talk about any conditions for the cease-fire.”

But Associated Press journalists on the border have seen the rebels with a wide range of unmarked military equipment — including tanks, Buk missile launchers and armored personnel carriers — and have run into many Russians among the rebel fighters. Ukraine also captured 10 soldiers from a Russian paratrooper division Monday around Amvrosiivka, a town near the Russian border. In videos posted on Facebook by Ukraine, one captive soldier said he did not know they were heading on a mission into Ukraine, while another said he did.

Those 10 have been taken to Kiev for questioning — but Ukrainian officials say Russia has not contacted them about the soldiers.

Ukraine wants the rebels to hand back the territory they have captured in eastern Ukraine, while Putin wants to retain some sort of leverage over the mostly Russian-speaking region so Ukraine does not join NATO or the European Union. Putin has so far ignored requests from the rebels to be annexed by Russia.

In Moscow, Denis Pushilin, one of the leaders of the pro-Russia insurgency, told reporters he had no information about whether Russian soldiers had entered Ukraine near Novoazovsk. But he said the Ukrainian separatists have been joined by many volunteers, including ones from Russia and Serbia.

AP reporters in eastern Ukraine have heard a variety of Russian accents from all over the country among the rebel fighters.

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Jim Heintz in Kiev, Ukraine, Nicolae Dumitrache in Donetsk, Monika Scislowska in Warsaw, Poland, and Lynn Berry in Moscow contributed to this report.