Haggard but relieved after days of fierce fighting with pro-Russian rebels, Ukrainian soldiers on Wednesday retreated on foot and by truck from the battleground town of Debaltseve.

Covered in the grime and soot of battle the government troops arrived at the nearest Kiev-held town of Artemivsk -- some 35 kilometers away (22 miles) -- exhausted and hungry.

Ukraine's President Petro Poroshenko on Wednesday called for a U.N.-mandated peacekeeping force to monitor a tattered ceasefire deal in east Ukraine.

"We see the best format would be a police mission from the European Union. We are sure this would be the most effective and best guarantee for security," Poroshenko said at the start of a meeting with security chiefs. 

Canada announced Tuesday new sanctions against Moscow and its sympathizers, including the state oil giant Rosneft, as pro-Russian separatists defied a ceasefire and stormed a flashpoint town in eastern Ukraine.

Two months after sanctions took aim at Russian interests in its vital oil and gas sectors, Canada targeted a flagship of the Russian economy in Rosneft.

Ukraine on Tuesday appealed to the West to get "tough" on Russia after separatists it says are militarily backed by Moscow stormed a flashpoint town to attack thousands of troops in violation of a three-day-old ceasefire.

As EU and U.N. officials expressed concern at the violence in the town of Debaltseve, Russian President Vladimir Putin -- who denies having a hand in Ukraine's insurgency -- urged Kiev to permit its beleaguered soldiers to surrender.

French Foreign Minister Laurent Fabius said Tuesday that "fundamental" parts of the ceasefire deal in Ukraine were not being followed, despite a reduction in fighting between government and rebel forces. 

"We can say the ceasefire is being generally respected, but there are two major 'buts'," Fabius told the French parliament. 

Vitaliy Karpunets crawls under barbed wire before jumping over a smoking bonfire into a trench as he fires off his Kalashnikov at an imaginary enemy.

At a military training center some 60 kilometers (40 miles) north of Kiev, Karpunets, 40, and some 5,000 other recruits are being put through their paces.

World powers voiced concern over a shaky truce in Ukraine that went into a third day Tuesday with no sign of a let-up in fighting for a key eastern town and no pull-back of heavy weapons from the frontline.

The United States and Germany called for a stop to the hostilities and urged free access for OSCE monitors blocked from entering the hotspot town of Debaltseve, a fiercely fought-for transport hub between the rebel-held cities of Donetsk and Lugansk.

The leaders of Russia, Germany and Ukraine have agreed on "concrete measures" to allow OSCE observers to monitor the shaky ceasefire in Ukraine, Berlin announced on Tuesday.

The measures were agreed during a late Monday phone call between German Chancellor Angela Merkel and her Russian and Ukrainian counterparts, Vladimir Putin and Petro Poroshenko, the government spokesman said in a statement, without providing details of the agreed steps.

Street-to-street fighting broke out for the first time inside a flashpoint town in east Ukraine on Tuesday, officials and pro-Russian rebels said, dealing a harsh blow to a fragile ceasefire just three days old.

The combat in Debaltseve, a strategic railway hub located between the separatist-held cities of Donetsk and Lugansk, was described as "intense".

President Francois Hollande, German Chancellor Angela Merkel and Ukrainian President Petro Poroshenko said Monday they were "concerned" about post-ceasefire fighting in the eastern Ukrainian town of Debaltseve, the French president's office said.

The three leaders, who spoke by telephone, "also expressed their desire for observers from the Organization for Security and Cooperation in Europe to have free access... to implement the ceasefire."