DONETSK, Ukraine (AFP) ―Ukraine said Monday its forces were closing in on the main rebel-held bastion of Donetsk and urged the insurgents to let civilians leave besieged eastern cities as fears of a humanitarian crisis mounted.

The military advances came as more remains from the downed MH17 plane were flown to the Netherlands for identification and Malaysian experts joined the Dutch and Australian probe at the site of the July 17 crash.

“Yesterday, forces from the Ukrainian antiterrorist operation occupied the city of Yasinuvata, 19 kilometers north of Donetsk, which is an important railway hub,” Ukrainian security spokesman Andriy Lysenko told journalists. 

“The seizure of the town allowed for the encirclement of Donetsk from the north and the closing down of an important channel for the supply of weapons and technology to the terrorists.”

He hinted that preparations for an offensive on the city were under way but gave no more details.

The Donetsk mayor’s office said active military operations were also going on in the town of Maryinka, not far from the outskirts of Donetsk, a city of one million in Ukraine’s industrial heartland.

Government forces have made major gains in the east over the past month and have said they are close to cutting off fighters in Donetsk from the Russian border and their comrades in Lugansk, the insurgents’ second largest stronghold.

Ukrainian troops continued to come under fire however, with Lysenko saying five soldiers had been killed and 14 more injured in recent days.

Kiev also reported it was negotiating the return of some 300 troops who were forced to retreat into Russia after hours of missile and mortar bombardment from across the border.

Russia’s security service said border guards had allowed the Ukrainians to cross after they agreed to give up their weapons.

More than three months of civil war have already claimed over 1,150 lives.

And Ukraine’s Defense Minister Valeriy Geletey admitted in a BBC interview Sunday “it would not be easy to liberate” Donetsk and Lugansk where rebels have pledged to fight to the death.

As fighting raged on, Kiev urged separatist rebels in Donetsk, Lugansk and another frontline city of Gorlivka to agree to “humanitarian corridors” for several hours each day to allow civilians to flee the besieged cities.

Civilians have borne the brunt of the conflict, with both government and pro-Moscow rebel forces accused of firing on builtup areas, resulting in more than 10 civilian deaths on the weekend.

The mayor of Lugansk has warned the city of 420,000 is on the brink of a “humanitarian catastrophe” after days without power or running water.

Over 100,000 people have fled for other parts of Ukraine since the fighting erupted in April, according to the United Nations, while Russia claims another half a million have crossed its border in search of refuge.

The exodus continued Monday with terrified residents waiting at Donetsk train station surrounded by piles of baggage.

“We are leaving because war is going on,” Igor, who was heading to the western city of Lviv with his elderly mother Nelya, said.

“Our house burnt down and our neighbor’s too. The whole street was bombed.

We are going to Moscow, to my daughter’s place,” added Yury Ivanovich, who was travelling with family members and neighbors.

Meanwhile, Malaysian experts joined the international probe into the July 17 crash of flight MH17 that killed all 298 people onboard, although the 100-strong team was briefly prevented from accessing the rebel-held site.

A plane carrying more remains of victims as well as DNA and belongings also flew back to the Netherlands for the painstaking identification process.

So far, over 220 coffins have been flown back the Netherlands ― which suffered the most casualties in the Malaysia Airlines plane crash.

But the search for more remains ― strewn across some 20 square kilometers ― continues under the baking summer sun.

The United States accuses insurgents of downing MH17 with a surface-to-air missile likely supplied by Russia, while Moscow and the rebels blame the Ukrainian military.

The U.S. and European Union last week slapped Moscow with the toughest sanctions since the collapse of the Soviet Union over its alleged support for the separatist rebellion.

But in a new move likely to alarm Ukraine, Moscow announced new military drills that will involve 100 aircraft on its southern flank.