Shelling in the eastern Ukrainian region of Donetsk killed at least six civilians Tuesday, as fighting intensified between government and rebel forces.

AP reporters saw the bodies of two people who were killed while waiting for a bus in the separatist-held city of Donetsk. Separatist authorities said another person died from an attack elsewhere in the city. Regional authorities loyal to the government said six people in total, including the three counted in Donetsk, had died throughout the day.

Fighting spiked over the weekend in the bitter battle for control over Donetsk's now-shattered airport. The level of artillery exchanges has abated since that peak.

In an effort to kick-start a peace process, German Foreign Minister Frank-Walter Steinmeier invited his counterparts from France, Russia and Ukraine to a meeting in Berlin on Wednesday to discuss the crisis in eastern Ukraine. Russian Foreign Minister Sergey Lavrov plans to attend, the ministry said in Moscow.

The resurgence in fighting during the last few days led "to a threatening situation," Steinmeier said in Berlin.

Negotiating stakes for the warring parties hinge on the terms of a cease-fire deal drawn up in September in the Belarusian capital, Minsk. Agreement was ostensibly reached over a line of contact between the opposing forces, although that has been insufficient to prevent continued fighting.

A new truce reached in December swiftly unraveled after the New Year and culminated with the confrontation that is still continuing at Donetsk airport and in surrounding areas. Separatists insist they are entitled to control over the terminal, which now remains little more than a smoldering shell, while Ukraine says the Minsk deal makes no provisions for the airport.

The Organization for Security and Cooperation in Europe, which was a party to the September deal, on Tuesday issued a declaration calling for an immediate, new cease-fire and for full implementation of the previous agreement.

At the World Economic Forum in Davos, Switzerland, Ukrainian President Petro Poroshenko said that de-escalation of the conflict will be the main topic at the meeting which will take place in Berlin on Wednesday.

He said that Ukraine intelligence reports show that around 2,000 troops entered Ukrainian territory and participated in combat operations.

"With this type of approach we will never ever reach de-escalation" he said.

Ukraine's military spokesman Andriy Lysenko earlier Tuesday said Russia this week boosted separatist numbers with two battalions, comprising 400 troops each. He didn't detail how that figure was obtained.

Russian defense ministry spokesman Maj. Gen. Igor Konashenkov called claims that troops had been sent into Ukraine as "absolute nonsense," according to Russia's Interfax news agency.

While Moscow has routinely denied supplying manpower and weapons to separatists in Ukraine, the sheer quantity of powerful arms in rebel hands has long strained those assertions. Numerous Russian citizens actively participate in combat in rebel formations, but Russia insists they are there on a voluntary basis.

The U.N. Office for the Coordination of Humanitarian Affairs cautioned earlier this month that the renewed intensification in hostilities is accompanied by an alarming deterioration of the humanitarian crisis. The U.N. has said it is concerned thousands of people are unable to buy essential food and medicine and that many are living in cold winter conditions.

More than 4,700 people have died since the unrest began, according to U.N. figures.

See entire Photographer Manu Brabo: Ukraine Unrest 2015 collection

Text from AP news story, Shelling in Eastern Ukraine Kills at Least 6 Civilians, by Mstyslav Chernov and Peter Leonard.

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