We could get a full-blown war in Ukraine this weekend.

SLOVYANSK, Ukraine (AP) — Ukraine launched an offensive against pro-Russian forces for control of a besieged eastern city Friday, while clashes between pro- and anti-government activists in the previously calm southern port of Odessa led to a fire that police said killed 31 people.

The first serious offensive by the government in Kiev and the dozens of deaths in Odessa sharply escalated the crisis that has led to the worst tensions between Moscow and the West since the Cold war. The Kremlin said the battle for the separatist-held city of Slovyansk effectively destroyed the Geneva pact aimed at cooling the unrest in the deeply divided country.

Oleksandr Turchynov, Ukraine’s acting president, said many insurgents were killed or wounded in the eastern offensive that also underlined the military’s vulnerability. The military action came two days after Kiev said it had lost control of eastern Ukraine.

Both sides said two Ukrainian helicopters were shot down by the insurgents near Slovyansk, killing two crew members, while authorities said another seven people also died: three separatist gunmen, two soldiers and two civilians.

By nightfall, Ukrainian troops and armored personnel carriers blocked all major roads into Slovyansk, and the central part of the city remained in the hands of pro-Russia gunmen, according to Associated Press journalists inside. Most shops were closed, and the few that were open were crowded with customers stocking up on supplies.

The Ukrainian Security Service said one helicopter was downed with a surface-to-air missile, adding that the sophisticated weapon undercut Russia’s claims the city of 125,000 people was simply under the control of armed locals.

There have also been violent clashes in Odessa, and at least three people have died in those.