He said at a briefing on Thursday, Censor.NET reports citing Reuters.
"We are reminded again of the continued challenge posed by Russia's aggression in eastern Ukraine," Tusk told a news conference. "The fighting must stop immediately. The ceasefire must be honored."
"Russia should use its influence to disengage the Russian-backed separatists."
The latest spike in violence in east Ukraine - where Russia-backed rebels face off against Kyiv government troops - coincides with the taking office by the new U.S. President Donald Trump, who has promised to seek rapprochement with Moscow.
Some Western media believe that recent escalation in the Donbas, namely in Avdiivka, is connected with Trump taking his post and his intentions of rapprochement with Russia.
The fighting in the vicinity of Avdiivka has been underway since late Saturday, Jan. 28.
After three days, Ukrainian soldiers managed to take enemy positions and improved their tactical set-up in the area. Unfortunately, casualties were reported among the military.
On Jan. 30, enemy strikes left the government-controlled town of Avdiivka without electricity, water and heat.
In the evening of Jan. 30, a state of emergency was declared in Avdiivka.
As reported by the anti-terrorist operation (ATO) Staff press center, militants carry out not stop attacks on Ukrainian positions near the town.
A regional coordination center to aid local population has been set up.
According to the National Police, six civilians had been wounded and 26 houses damaged as of morning, Feb. 1. A female body of a civilian was found in Avdiivka downtown on Feb. 1.
The Donetsk regional civil-military administration said the sides had agreed to cease fire until 5 p.m. Feb. 1. At around 3:30 p.m., the enemy violated the agreement by opening fire on repair workers. The town remains de-energized so far.
Related materials: Avdiivka fighting
Russian proxies mortared Avdiivka residential area, civilian wounded, - Donetsk governor Zhebrivskyi(0)
Avdiivka residential area comes under mortar attack, civilian injured, - Donetsk governor Zhebrivskyi(0)