The foreign ministers of Germany, France, Russia, and Ukraine are due to meet on January 21 to discuss the conflict in eastern Ukraine, where there has been a spike in fighting in recent days.
The scheduled meeting in Berlin comes a day after the Ukrainian military said its forces came under attack from Russian regular forces at two checkpoints northwest of the city of Luhansk.
Moscow had no comment.
Ukrainian President Petro Poroshenko is cutting short his stay at the World Economic Forum in Davos, Switzerland, to monitor the situation at home.
A Ukrainian military spokesman also said two "battalion tactical groups," each of about 400 men, had crossed into Ukraine from Russia -- a charge dismissed as "absolute nonsense" by the Russian Defense Ministry.
On January 19, Ukrainian Prime Minister Arseniy Yatsenyuk said that Russian-made rocket and weapons systems were pouring into the country.
In announcing the meeting on January 20, German Foreign Minister Frank-Walter Steinmeier said the resurgence in fighting during the last few days has led "to a threatening situation."
"The chief aim now is to prevent a further deterioration of the military conflict and a renewed political escalation between Kyiv and Moscow," Steinmeier said in a statement.
He will host his Russian counterpart Sergei Lavrov, Ukraine's Pavlo Klimkin, and Laurent Fabius of France.
The group last met in Berlin on January 12.
Fighting spiked over the weekend in the bitter battle for control over the now-shattered airport in the city of Donetsk, a rebel stronghold.
The level of artillery exchanges has abated since that peak.
Kyiv claims its forces have regained control over much of the airport, something the pro-Russian separatists deny.
The Organization for Security and Cooperation in Europe (OSCE) on January 20 issued a declaration calling for an immediate, new cease-fire and for full implementation of a cease-fire deal drawn up in September in the Belarus capital, Minsk.
Klimkin said on January 20 that the pro-Russian rebels had seized more than 500 square kilometers of territory from Ukrainian forces.
Klimkin said the rebels, backed by Russian armament and soldiers, had taken "advantage of the fact that our forces complied with the cease-fire."
The International Committee of the Red Cross (ICRC) has expressed deep concern over the escalating fighting in eastern Ukraine.
In a January 20 statement, the ICRC said the fighting in and around the city of Donetsk is killing civilians and is "preventing" its team from carrying out its humanitarian work.
The Red Cross said "harsh winter conditions and the suspension of benefit payments" in some parts of the Donetsk and Luhansk regions are making the situation "even more difficult."
It said it is distributing food, hygiene items, medical supplies, and construction materials to both residents and displaced people in the two regions, which are partly held by the rebels.
The conflict in eastern Ukraine began last April after pro-Russian separatists took control of parts of the Donetsk and Luhansk regions following Russia's illegal annexation of Crimea from Ukraine in March.
The annexation came after Moscow-backed Ukrainian President Viktor Yanukovych fled in February following months of antigovernment protests in Kyiv over his November 2013 decision to spurn a major pact with the EU.
More than 4,700 people have died in the fighting, according to UN figures.
With reporting by Reuters, AP and AFP