Defense Ministry spokesman Oleksiy Dmytrashkivsky said Tuesday that Ukraine had unleashed a "massive artillery and air offensive" following the end of a ceasefire.

After declaring a weeklong truce in June and then extending that agreement through Monday, Ukrainian President Petro Poroshenko had evidently rejected the idea of waiting any longer for the separatists to begin peace efforts.

Russian officials immediately expressed "deep regret." President Vladimir Putin said Poroshenko would assume responsibility for future casualties and gathered his top security aides to discuss the "swiftly deteriorating" situation in Russia's western neighbor.

Over the weekend and again on Monday, German Chancellor Angela Merkel and French President Francois Hollande held teleconferences with Poroshenko and Putin. The two leaders found themselves in rare agreement with Putin, and urged Ukraine's president to extend the truce to give indirect talks between separatist commanders and the government a chance.

The European Union, United States and Ukrainian government have accused Russia of providing financial and logistical support to the separatists. They have introduced sanctions against Russia and threatened more.

'My main goal'

In a dramatic midnight address immediately after the ceasefire expired, Poroshenko told Ukrainians that he had allowed the agreement to end because separatists had used it to regroup and stock up on heavy arms from Russia.

Poroshenko also accused the rebels of attacking government positions more than 100 times during the truce. On Monday, Ukraine's Foreign Ministry posted on Twitter that during the ceasefire separatists had killed 27 members of the military and wounded 69 more.

"Peace has been and will remain my main goal," the 48-year-old president said. "Only the means to achieve it have changed."

On Tuesday, both separatist fighters and Ukrainian troops reported heavy exchanges of artillery fire and air bombardments across the separatist strongholds of Luhansk and Donetsk on the Russian border.

Poroshenko said his forces had seized back one of 24 border crossing in the Luhansk region. On Tuesday, he congratulated the troops on their "first victory." However, most of the other roads into Russia remain firmly under separatist control.

The 12-week conflict has killed about 450 people, including at least five journalists. It has also displaced thousands.

mkg/slk (Reuters, AFP, dpa, AP)