NATO Secretary General Anders Fogh Rasmussen will visit Kiev Thursday, Ukraine's foreign ministry said, amid heightened tensions over allegations Russia is massing troops along its ex-Soviet neighbor’s border.

The visit, at the invitation of Ukrainian President Petro Poroshenko, is intended to discuss an upcoming meeting on NATO-Ukraine partnership, the foreign ministry said Wednesday.

A first air strike hit the main rebel stronghold of Donetsk in east Ukraine Wednesday, as government forces preparing to retake the city said 18 soldiers were killed in clashes with rebels.

An "air strike" was carried out on the main rebel stronghold of Donetsk in east Ukraine overnight but caused no civilian casualties, local authorities said Wednesday. 

As Western countries launch each new salvo of sanctions against Russia over the Ukraine crisis, Moscow has returned fire with bans on food imports -- ostensibly for consumer protection.

Milk, cheese and onions from Ukraine, Greek peaches, Serbian prunes, Polish apples and cabbage, Spanish meat -- according to Russia's food safety regulator Rosselkhoznadzor all these products contain toxic elements, are infected with dangerous bacteria or violate other regulations.

For a few hours at least, the road leading west from the besieged rebel stronghold of Donetsk was meant to provide a sliver of security for desperate residents trying to flee the ever-encroaching fighting in east Ukraine.

But a mortar blast just yards from the road and the booms of fierce clashes between government troops and insurgent fighters in a town along the route showed that even this "humanitarian corridor" announced by Kiev on Monday was hardly safe.

At least two civilians were killed in heavy clashes on the outskirts of the main rebel-held bastion of Donetsk in east Ukraine on Tuesday, as the U.N. warned of a "massive exodus and destruction" if fighting intensified.

Powerful explosions rocked the western Petrovksy suburb of the sprawling million-strong city, the local administration said, as government forces pushed on with an offensive to oust insurgents.

With a scarred face and eyes red with fatigue, Ukrainian serviceman Pavel sits in a tent on the Russian side of the border, contemplating a decision he already regrets.

"We were given the order to surrender," he said wearily. "Orders are orders, but this time, I did not want to obey."

Terrified residents on Tuesday fled the besieged rebel bastion of Donetsk in eastern Ukraine along a perilous humanitarian corridor, as government forces pushed their offensive to the outskirts of the city.

Dozens of cars packed with civilians headed along a road out of the pro-Russian stronghold, past the town of Maryinka where explosions from continuing clashes rumbled on.

Pro-Kremlin rebels in eastern Ukraine are hampering the treatment of a growing number of civilian casualties by threatening doctors, hijacking ambulances and looting medical equipment, Human Rights Watch said Tuesday.

Russia said on Monday that it will be easier to deal with the incoming new U.S. ambassador than with his "freelance" predecessor who provoked Moscow's wrath with his support for the opposition.

John Tefft, the former U.S. ambassador to Kiev, was approved as Washington's new envoy by Moscow last month. His predecessor, Stanford professor Michael McFaul, quit his post in February only two years into the job.