Ukrainian forces took control of Sloviansk on Saturday, a city in the east of the country that had been held for months by separatists seeking to join Russia.

Ukrainian officials touted the army’s victory as a major step forward in their attempt to pacify southeastern Ukraine while the rebels acknowledged the loss of the city.

Aleksandr Borodai, a leader of the breakaway Donetsk region that declared itself independent in April, was quoted by the Russian Interfax news agency as saying, “Given the disproportionate numerical superiority of the enemy troops, units of the armed forces of the Donetsk People’s Republic were forced to leave their previous positions on the northern sector of the front.”

Donetsk is at the heart of a rebellion that opposes centralized rule from Kiev and wants Russia to annex parts of the predominantly Russian-speaking region.

Sloviansk was possibly the militants’ strongest redoubt. Its loss marks a major setback for the insurrection after three months of fighting and a welcome boost for Ukraine’s president, Petro Poroshenko, who was elected last month on a promise to put down the uprising.

More than two hundred Ukrainian troops have been killed since the conflict broke out. Hundreds more civilians and rebels are believed to have lost their lives.

Poroshenko declared a unilateral ceasefire two weeks ago but refused to extend it on Monday, citing numerous violations by the rebels. Instead, he launched a fresh offensive in the east that quickly drew the ire of Russia.

While President Vladimir Putin last week formally renounced the right to intervene militarily in Ukraine, which his parliament had granted him in March, Russia has acted as the rebels’ patron. It denies Western accusations that it has backed the separatists with a view to dismembering the former Soviet republic. But Russian weapons do appear to have found their way into the country while Russia’s proposal to end the crisis — turning Ukraine into a federation to give Russian speakers in the southeast more of a direct say in how they are governed — is considered a ploy to divide the country by Ukraine’s authorities.

Earlier this year, Russia invaded Ukraine and annexed the Crimea after its residents, most of whom are of Russian descent, voted overwhelmingly in favor of joining Russia in a referendum. The peninsula had belonged to Ukraine since 1954 when the country was part of the Soviet Union.

The rest of the southeast also has a large ethnic Russian minority but opinion polls have consistently found low support for independence or becoming part of the Russian Federation, notwithstanding skepticism of the more pro-Western government in Kiev.

Ethnic Russians and Russian speakers in the east were rankled when demonstrations in the west of the country forced former president Viktor Yanukovich to resign after he pulled out of an association agreement with the European Union at the last minute. Yanukovich fled to Russia which had tried to dissuade Ukraine from entering into the association agreement, seeing it as tantamount to the country moving into the Western sphere. Poroshenko’s government signed the agreement, which commits Ukraine to the gradual approximation of its economic, judicial and security policies to those of the European Union, last month.

The treaty also gives Ukraine European financial support and will eventually establish a free-trade area between the country and its neighbors. It is seen as a stepping stone to European Union membership.