Fighting continues in eastern Ukraine as foreign ministers prepare for talks in Berlin

Germany's foreign minister said Wednesday that a ceasefire was a condition for resolving the Ukraine crisis, ahead of four-way talks hastily called as the ex-Soviet state pursued its offensive against pro-Kremlin insurgents.

Describing the situation in eastern Ukraine as "very dangerous," Frank-Walter Steinmeier said fighting in the region had "dramatically intensified" as he readied to host the talks with his French, Russian and Ukrainian counterparts.

Ukrainian President Petro Poroshenko on Monday took the risky step of contradicting his European allies for the first time since his May 25 election by ripping up a 10-day truce.

"An explosion of violence can occur at any time that cannot be controlled either politically or militarily," Steinmeier warned.

"Only when the guns fall silent, only on the basis of a robust ceasefire, are negotiations on resolving the crisis imaginable."

Steinmeier, who has been highly active in trying to ease tensions in Ukraine since the crisis began in February, stressed the need to grasp opportunities to prevent a further escalation.

He said Russia and Ukraine's attendance at the Berlin talks later Wednesday was "an important and a good sign" but said that achieving a lasting truce would be "anything other than an easy undertaking."

Meanwhile, Ukrainian government forces pressed on with a military drive against pro-Russian separatists. 

Rebels fired a shoulder-launched missile that struck and damaged a SU-24 attack plane, a military spokesman said, while one Ukrainian border guard was killed in the early hours in a mortar attack on his post on the border with Russia.

"The armed forces and the National Guard are continuing the offensive on terrorists and criminals. The actions of our military are effective and are having results," parliament speaker Oleksander Turchynov said.