UN Secretary-General Ban Ki- moon reiterated that military means alone can't provide solution to the Ukrainian crisis and that a renewed and lasting ceasefire upheld by all parties is fundamental, his spokesman said on Monday.

"The secretary-general is aware of reports that armed militias have regrouped in Donetsk and Lugansk and that the Ukrainian forces are engaging in intensified military activity," Ban's spokesman said in response to a press question.

"The secretary-general reiterates that military means alone cannot provide a solution. A renewed and lasting cease-fire upheld by all parties is fundamental," the spokesman said. "The continuation of a political and diplomatic process towards a definitive cessation of violence and a peaceful resolution of the crisis is critical."

"The secretary-general strongly condemns the continued, unlawful actions of armed groups," the spokesman said. "He also urges the Ukrainian authorities to exercise maximum restraint and to take every possible step to ensure the protection of the large civilian population in these areas."

Three bridges have been destroyed on roads leading into the city of Donetsk on Monday ahead of an expected offensive from Ukraine's government forces, reports said, adding that Ukrainian troops regained control of the key rebel strongholds of Sloviansk and Kramatorsk on Saturday.

Ukrainian troops and rebel militants exchanged mortar fire on Saturday night near the Lugansk airport controlled by the Ukrainian law-enforcers, the reports said.