German Chancellor Angela Merkel has called on President Vladimir Putin to explain reports alleging Russian troops and tanks had advanced into eastern Ukraine to support the separatist rebels in the area.
This, as Pavlo Klimkin, Ukraine's foreign minister, pleaded for military aid from the European Union and NATO.
Alexander Zakharchenko, prime minister of the self-proclaimed "Donetsk People's Republic," said in reports last week that it was Russia supplying his group with new armoured vehicles. Added to that are Russian-trained fighters.
Zakharchenko said in a video posted on the Internet, and quoted by the news agency AFP, that the new troops had had "four months of training on Russian territory" and had "been brought in at the most crucial moment."
Ms Merkel discussed the alleged reports with Ukrainian President Petro Poroshenko in a telephone conversation on Saturday.
If these were true, it is imperative that these be stopped, Steffen Seibert, Ms Merkel's spokesman, said in a statement.
Ms Merkel and Mr Poroshenko likewise agreed a cease-fire must ensue, Seibert added.
The relentless killing between government and separatist rebels has killed over 2,100 people. Now running for four months, the tragedy has unleashed a humanitarian crisis.
Overnight, separatist rebels had shot down another Ukrainian warplane in the Luhansk region of eastern Ukraine. Since June, at least 10 Ukrainian aircraft have been targeted and brought down by the separatists.
Russia's Interfax news agency quoted separatist leader Andrei Sakhartshenko as saying they are now targeting the Ukrainian army blockade of the city of Luhansk.
Meantime, Klimkin, Ukraine's foreign minister, pleaded with the EU and NATO to send the much-needed military aid to help resolve the situation and for peace to come to Donetsk and Luhansk.
While he lauded Germany and Ms Merkel for taking the middleground, Klimkin said there's much more to be done to defuse the conflict.
"The Chancellor is doing an unbelievable great deal. She is one of the most important leaders of this process. She has done a lot for the European Union to become univocal. And for that, we are incredibly grateful. But it goes without saying that more needs to be done " Klimkin said.
"We need military aid. Because if such aid comes, it will make it easier for our subdivisions to work on the ground," RIA Novosti quoted Klimkin.