SARTANA, Ukraine -- Fighting intensified Tuesday in eastern Ukraine as pro-Russia rebels and Ukrainian troops sought to extend their gains ahead of crucial peace talks, and the government accused the separatists of shelling a town far behind the front lines, killing 12 people and wounding scores.
Germany, which has joined with France to try to broker a peace deal, urged Russia and Ukraine to compromise and called on the warring parties to refrain from hostilities that could derail a four-way summit Wednesday in Minsk, Belarus.
Ukrainian President Petro Poroshenko said the talks were "one of the last" opportunities for ending the fighting that has killed more than 5,300 people since April.
Poroshenko told parliament in Kyiv that the separatists launched a rocket strike Tuesday on the town of Kramatorsk, more than 50 kilometres from the nearest front line, hitting a regional military command centre with the first salvo and then striking a nearby residential area.
Rebels denied any involvement in the attack and said it was a "provocation" by the Ukrainian authorities. Kramatorsk was the site of major fighting until July, when pro-Russian separatists retreated.
The government-controlled Donetsk administration said 12 people were killed by the barrage in Kramatorsk and 64 were wounded, including 29 civilians. Photos on the local Donetskiye Novosti website showed an artillery shell embedded in the ground next to a residential building and two bodies lying nearby.
Further south, the volunteer Azov battalion, loyal to the government in Kyiv, said on social media Tuesday that it captured several villages northeast of the strategic port of Mariupol, pushing the rebels closer to the border with Russia.
However, a rebel spokesman, Eduard Basurin, insisted in a televised news conference that the rebels had not retreated.
The pro-government volunteers said rebels shelled the village of Kominternove, east of Mariupol, causing unspecified civilian casualties. An Associated Press reporter at a government checkpoint between there and Mariupol heard of fighting several miles away and saw two ambulances and four trucks carrying Ukrainian troops from the direction of Kominternove toward Mariupol.
Oleksandr Turchynov, chairman of the Ukrainian Security and Defence Council, visited the area Tuesday and said the government offensive aimed to bolster the defences of Mariupol and "protect civilians from artillery strikes."
The rebels reported advances, too. Basurin said they had surrounded the railway hub of Debaltseve, the focus of fierce fighting in the past weeks, cutting it off from a major highway.
At least seven Ukrainian troops were killed overnight in the east, military spokesman Anatoliy Matyukhin said, while in the rebel stronghold of Donetsk, which faces frequent shelling, two civilians were reported killed and 12 injured.
Amid the escalating hostilities, the insurgents announced a call-up of new volunteer recruits in Donetsk, following a pledge by separatist leader Alexander Zakharchenko to strengthen his forces to 100,000 men. Rebel official Arkadiy Fedoseyev said the separatists were recruiting "tank drivers, mechanics, technicians and repairmen."
One 18-year-old volunteer, who gave only his first name, Nikolai, said he had come from the Russian city of Volgodonsk.
"I have no parents. I'm from an orphanage. I saw on television how other people's parents are being killed and I decided that this was not good," he said, adding that he wants to be a sniper.
The mounting death toll comes amid renewed efforts to work out a peaceful solution in a conflict that has displaced at least 1 million people and left the Ukrainian economy in ruins. Ukraine and the West accuse Russia of fueling the rebellion with troops and weapons. Moscow denies the charge, but the sheer number of sophisticated heavy weapons in the rebel hands belies the denial.
Ahead of Wednesday's talks, President Barack Obama spoke by phone with both Putin and Poroshenko.
In the call to Putin, Obama reiterated U.S. support for Ukraine's sovereignty and emphasized the importance of reaching a diplomatic resolution.
"However, if Russia continues its aggressive actions in Ukraine, including by sending troops, weapons, and financing to support the separatists, the costs for Russia will rise," the White House said in a statement.
The Ukrainian leader's office said Poroshenko and Obama expressed hope that Wednesday's summit would bring a halt to fighting.
"Tomorrow's meeting in Minsk offers one of the last chances to declare an unconditional cease-fire and pull back heavy artillery," Poroshenko said Tuesday.
Representatives of Ukraine, Russia, the rebels and the Organization for Security and Cooperation in Europe met Tuesday to lay the groundwork for Wednesday's summit in Minsk, and senior officials from the four nations were holding parallel talks.
"We are going to Minsk with the firm will to succeed, without being certain that we can do so," French President Francois Hollande said, adding that he and German Chancellor Angela Merkel were committed to doing everything possible to achieve "an agreement, a global settlement."
Hollande said the deal was also essential for economic reasons, "so that there can be a strong resumption of trade," which was hurt by Western sanctions against Moscow and Russian counter-sanctions.
Amid the worst Russia-West crisis since the Cold War, France suspended delivery of a navy ship it built for Russia until progress was made toward a peaceful resolution in Ukraine.
German Foreign Minister Frank-Walter Steinmeier spoke by phone with his Russian and Ukrainian counterparts to push for compromise.
"We have a great opportunity again tomorrow to take a first, important step toward defusing the conflict, hopefully toward silencing the weapons," Steinmeier said. " I hope that none of those involved in the fighting will push things so far that Minsk is called into question by an explosion of violence in the final hours."
The Kremlin warned the West against sending weapons to Ukraine or putting pressure on Russia.
Russian President Vladimir Putin's spokesman, Dmitry Peskov, told the Russian News Service radio station that any talk about imposing new sanctions on Russia or arming the Ukraine government would destabilize the situation.
In Washington on Monday, Obama and German Chancellor Angela Merkel rallied behind efforts to reach a diplomatic resolution to the conflict but offered no clear path for how the West would proceed if the talks in Minsk failed.
Merkel and other European leaders oppose arming Ukraine's beleaguered military but Obama dangled the prospect that the U.S. could, for the first time, send lethal defensive weapons to Ukraine.
