Ukraine’s interim government took additional steps Tuesday to reassert its control by appointing a new military commander and shoring up security forces, even as some leaders made urgent calls for volunteers to take up arms against pro-Russian separatists in the eastern part of the country.

Amid reports of fresh violence, former prime minister Yulia Tymoshenko urged the creation of a “volunteer army” because neither Ukraine’s army nor its security services have been effective in handling outbreaks of rebellion, the Russian news service Interfax reported Tuesday.

Hers was one of many calls to form combat-ready units of “self-defense” forces ahead of May 25 presidential and mayoral elections. Andriy Tiron, battalion commander of the National Guard, told reporters in Kiev that demonstrators who helped oust the previous pro-Russian government were being urged to volunteer for military duty. But there was confusion about who would command them and what their duties would be.

In a brief statement that appeared on his official Web site, the acting president, Oleksandr Turchynov, said Lt. Gen. Anatoly Pushnyakov has been appointed to take command of the army. The statement offered no further details. Turchynov also published a decree sacking the top regional administrator in Odessa as violence continued to flare around eastern Ukraine.

Late Tuesday, there were reports of fighting in the eastern coastal town of Mariupol. Interfax, the Russian news service, quoted a separatist representative as saying that Ukrainian forces had staged an attack. Local media reported cars ablaze and a skirmish underway near the Mariupol airport.

The fight for the eastern Ukrainian city of Slovyansk continues to unfold, despite mounting losses, mostly on the side of pro-Russian separatists. (Reuters)

Earlier, in the Luhansk region, about 20 armed militants destroyed a military radar station, according to the Web site of southern Ukraine’s prosecutor general. The report said the militants outnumbered Ukrainian soldiers at the installation and slipped away after the 4 a.m. attack. There was no mention of casualties. In perhaps another sign of the confusion and suspicions of mixed loyalties in parts of Ukraine, the prosecutor said there would be an investigation of the soldiers’ conduct.

Armed separatists also seized control of the main government building in the town of Debaltseve, a regional railway hub with a population of 26,000 and two large thermal power plants about 45 miles northeast of Donetsk, according to local media reports.

In Odessa, social media appeared to be fanning tensions between pro-Russian and pro-Ukrainian activists days after 46 people died in violent clashes in the city. The Kyiv Post said late Monday that the rival groups have been using VKontakte — the Russian counterpart of Facebook — to rally supporters for demonstrations and counterdemonstrations and to single out individuals for reprisals.

Acting interior minister Arsen Avakov, Ukraine’s top security official, said Tuesday that an estimated 30 pro-Russian militants were killed and “dozens” were injured in fighting a day earlier around the eastern city of Slovyansk. Four members of Ukraine’s security forces were killed and about 20 were wounded as the interim government attempted to take control of the city.

A Ukrainian diplomat charged Tuesday that Russian “meddling” in Ukraine appeared aimed at disrupting the elections, which are less than three weeks away. But Danylo Lubkivsky, deputy minister for foreign affairs, said the interim government remained intent on ensuring that ballots would be cast in all parts of the country. He also reiterated the government’s interest in talking with regional leaders in rebellious areas about more autonomy for those regions.

“However, we will not engage in negotiations with terrorists,” Lubkivsky said at a news conference in Kiev. He also called the deadly clashes in the port city of Odessa “an unspeakable tragedy” and blamed the spread of the violence on Russia.

German Foreign Minister Frank-Walter Steinmeier on Tuesday called for a second round of talks in Geneva. Russian officials also called for further talks but said pro-Russian separatists should be at the table.

Map: Russia and Ukraine are positioning their troops for war.

“Getting together again in the same format, with the opposition to the current Ukrainian regime being absent at the table of negotiations, is unlikely to add anything,” Russian Foreign Minister Sergei Lavrov said. “It is possible, of course, but we shall be going in circles,” he told reporters at the Council of Europe in Vienna, Interfax reported.

Lavrov said Ukraine’s plan to hold elections on May 25 was “highly unusual” amid a military operation aimed at regaining control over the east.

The chairman of the Organization for Security and Cooperation in Europe is expected to visit Moscow on Wednesday in an attempt to foster more negotiations. Russian officials have said they would like the OSCE to be the intermediary for any discussion about Ukraine’s future.

Meanwhile, a pro-Ukrainian activist, Nikolay Yakubovich, who was taken hostage by separatists on Thursday, was exchanged for an unspecified number of separatist prisoners on Tuesday, according to local media.

Human Rights Watch said the fate of at least two dozen other people taken captive by pro-Russian separatists remained unknown.

Denyer reported from Slovyansk. Birnbaum reported from Moscow. Alex Ryabchyn in Donetsk and Anna Nemtsova in Odessa contributed to this report.