KIEV (AFP) ― NATO chief Anders Fogh Rasmussen urged Russia to “step back from the brink” during a visit to Kiev on Thursday and vowed support for Ukraine as fears mounted that Moscow was preparing to send troops into the conflict-torn east of the country.

His visit came as the center of the main rebel bastion Donetsk was hit by sustained bombardment for the first time in four months of fighting, and a dozen civilians were killed around the region.

An AFP journalist also saw a Ukrainian fighter jet shot down as it flew low over rebel-held territory. The plane crashed after an explosion in midair and the pilot was seen parachuting out.

Amid heavy fighting and political maneuvering, Kiev scrapped a cease-fire around the crash site of Malaysia Airlines flight MH17, a day after international experts suspended work there due to ongoing clashes.

Russia meanwhile struck back at punishing Western sanctions by announcing a “full embargo” on foods from the United States and the European Union and threatening to ban flights from crossing its air space.

Rasmussen’s visit to Kiev came just a day after NATO accused Moscow of massing some 20,000 troops on the border with its former Soviet neighbor.

After meeting with Ukraine’s leadership on Thursday, Rasmussen assured them that “NATO stands ready to support Ukraine with advisers and assistance” against what he called “Russian aggression.”

“Today, Ukraine’s freedom and future are under attack. Instead of de-escalating the conflict, Russia continues to destabilize Ukraine,” he said.

The West accuses Russia of supporting and instigating the insurgency in eastern Ukraine, sending tensions with Moscow soaring to their highest point since the Cold War.

“I call on Russia to step back from the brink, to step back from the border. Do not use peacekeeping as an excuse for warmaking,” Rasmussen added, calling on Moscow to “stop its support for separatists.”

NATO has warned that Russia could invade Ukraine under the pretext of a “humanitarian” mission, but Moscow has dismissed claims it has ratcheted up troop numbers.

Fighting raged on in Ukraine’s industrial east, where local authorities have warned of a looming humanitarian catastrophe. Certain areas have been left without water or power and hundreds of thousands have already fled.

In Donetsk, health authorities said one person was killed and two injured when a mortar hit a central hospital, as bombardments peppered the area around a key rebel base.

Two civilians were also killed by shelling in a western suburb of the city, the local administration said.

Government forces have been gearing up to retake Donetsk in a bid to cut it off from the Russian border and the second largest rebel-held city of Lugansk.

Heavy shelling of Gorlivka, another rebel bastion just north of Donetsk, killed five and wounded 10, adding to three more casualties overnight further south, local authorities said.

So far, over 1,300 people have been killed and 285,000 have fled in four months of what the Red Cross has designated a civil war.

The National Security and Defense Council said seven soldiers had been killed and 19 injured overnight.

A military spokesman confirmed that rebels shot down a Ukrainian fighter jet and said a search-and-rescue operation was underway for the pilot.

Ukrainian forces have lost a string of aircraft during the conflict.

The plane came down some 40 kilometers west of the crash site of the MH17 passenger jet. The U.S. has accused rebels of shooting down the plane at an altitude of some 10,000 meters with a missile likely supplied by Russia.

Despite the fierce fighting, diplomatic efforts to bring an end to the fighting were ongoing on Thursday.

A spokesman for the German government said Chancellor Angela Merkel discussed “practical ideas” to help ceasefire efforts during a phone call with Ukraine’s President Petro Poroshenko.

The two also discussed the need for a resumption of the search around the MH17 crash site.