Pro-Russian separatists in Ukraine say they have evacuated their eastern stronghold of Sloviansk amid a large-scale offensive by the military.
Defence Minister Valeriy Heletey said the Ukrainian flag was flying over the city council building.
Separatist leaders said fighters had relocated in the face of the army’s “overwhelming numerical superiority”.
Ukrainian forces launched an offensive against the separatists this week after a 10-day ceasefire broke down.
The BBC’s David Stern in Kiev says the military’s capture of Sloviansk – where the eastern insurgency began in April – is a major victory for the government.
Sloviansk’s rebel-backed mayor, Vladimir Pavlenko, confirmed to Agence France-Presse news agency that the separatists had left.
Alexander Borodai, prime minister of the self-proclaimed Donetsk People’s Republic, said on a rebel website: “Due to the overwhelming numerical superiority of the enemy our men were forced to abandon their positions.”
The Associated Press quoted rebel spokesman, Andrei Purgin, as saying the army offensive had left the town “in ruins”.
Another rebel commander told AP the fighters were moving to the town of Kramatorsk.
Witnesses said there was bombing overnight in Sloviansk and in the morning the rebel-manned checkpoints had been abandoned.
Interior Minister Arsen Avakov earlier said on Facebook that the rebels were “suffering losses and surrendering” after leaving Sloviansk and being met by Ukrainian forces.
Mr Avakov said “some intelligence suggests” that the military commander of the self-declared Donetsk Peoples’ Republic (DPR), Igor Strelkov, was among those leaving.
The commander, whose real name is Igor Girkin, is accused by Ukraine’s government and the European Union of being a Russian military intelligence officer.
Who is Igor Strelkov?
- Real name Igor Girkin, military commander of separatist forces of self-declared Donetsk People’s Republic
- Ukraine says he works for Russian military intelligence agency the GRU
- On list of EU sanctions for posing threats to Ukraine’s independence
- Noted for love of re-enacting Roman, Napoleonic and 20th Century battles
Mr Poroshenko’s website said the separatists had lost one tank and four armoured vehicles as they left.
However, one rebel leader, Pavel Gubarev, said the losses suffered were “minimal”.
In a separate development, Russian news agency Interfax reported that Ukrainian forces had destroyed the headquarters of the DPR in the town of Artemivsk.
There has been intense diplomatic activity this week to try to reinstate the ceasefire, with telephone calls involving Germany, France, the US, Russia and Ukraine.
Mr Poroshenko said he was ready to return to a ceasefire provided it was observed by both sides, all hostages were freed and borders secured by government forces.
Our correspondent in Kiev says talks to try to end the fighting were supposed to begin on Saturday, but so far no meeting has been announced.
Human cost of conflict
At least 250 civilians killed in eastern Ukraine since April, according to Ukrainian and rebel reports
Nearly 200 soldiers killed and 619 wounded since April, according to Ukrainian defence ministry
At least 800 rebels killed since April, according to rebels
At least 110,000 people have left Ukraine for Russia this year to date, most of them from eastern regions, according to the UN
Some 54,400 people have been displaced within Ukraine itself, the UN says
Mr Poroshenko called off a unilateral truce on 30 June, accusing the rebels of staging deadly attacks on Ukrainian government troops.
Violence erupted in eastern Ukraine in April, when the separatists declared independence in the Donetsk and Luhansk regions.
That followed Russia’s annexation of Ukraine’s Crimea.
The current Ukraine crisis started last November, when then President Viktor Yanukovych decided, under pressure from Russia, not to sign an agreement with the EU, leading to street protests in Kiev and his eventual overthrow.
President Poroshenko signed the free trade part of the EU deal in Brussels on 27 June, after earlier signing the political co-operation clauses.