Ukraine will only negotiate with the rebels once they lay down their arms, Kiev's defence minister has said.
"There will be no more unilateral ceasefires" by Ukrainian troops, Valeriy Heletey said in a statement posted on the ministry's website.
He was speaking after separatists fled their eastern strongholds and regrouped in the regional centre of Donetsk.
Ukrainian forces recaptured several cities after President Petro Poroshenko brought an end to a 10-day ceasefire.
He has since ordered the army to blockade the insurgents inside the two cities of Donetsk and Luhansk. On Tuesday he named a new anti-terror chief, Vasyl Grytsak.
Mr Poroshenko had promised ceasefire talks no later than last Saturday, but that was before the series of recent military successes.
The latest ceasefire expired at the end of June and failed to lead to any negotiated settlement.
One of the rebel leaders in the east, Alexander Borodai, told Russian media they were preparing to fight back. "We're not preparing ourselves for a siege. We are preparing ourselves for action," he said.
A day after three bridges were blown up in the Donetsk area, Ukrainian website Ostrov reported that the rebels had targeted another railway line in the area.
Pictures of the destruction showed damage to the line between Yasynuvata and Horlivka, near Donetsk.
The authorities said they did not know who was responsible for the destruction of the bridges, although some witnesses were quoted as saying they saw men in pro-Russian rebel uniforms at one location.
A railway line in the village of Novobakhmutivka, which crosses over a main road out of Donetsk, was destroyed as a large cargo train was travelling on it.
A witness told the AP news agency that he saw a group of camouflaged men get into their cars and head towards Donetsk shortly before the bridge collapsed after an explosion.
However, Ukraine's security forces say they have thwarted one rebel attack, detaining five local men in the Zaporizhya region west of Donetsk on suspicion of planning to blow up a railway line, Interfax Ukraine reports.
They were caught reportedly carrying explosives and maps of bridges, and were thought to have been involved in an earlier attack on 24 June.
In the absence of a truce, European Union envoys are still expecting to meet in Brussels on Wednesday to consider imposing travel bans and other sanctions on more key Ukrainian individuals from both sides of the fighting in the east of Ukraine.
Separatists declared independence in the Donetsk and Luhansk regions of eastern Ukraine after Russia annexed the Ukrainian region of Crimea in March.
Moscow's move followed a decision in November 2013 by Ukraine's then President Viktor Yanukovych to pull out of an association deal with the European Union - sparking huge street protests that eventually led to his downfall.
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