Russian-backed forces in eastern Ukraine have launched an offensive to recapture a city on the border of the territory currently controlled by separatist groups, according to a U.S. diplomat.

"Russia and the separatists initiated the violence in Avdiivka," Kate Marie Byrnes, the charge d'affaires of the U.S. mission to the Organization for Security and Cooperation in Europe, said during an emergency meeting of the organization. "We call on Russia to stop the violence, honor the cease-fire, withdraw heavy weapons, and end attempts to seize new territory beyond the line of contact."

The State Department issued a travel warning for Ukraine in December, following reports of increased violence on the border of the contested territory, but the fighting intensified this week. OSCE reports that there were 2,499 explosions in a single 24-hour period from Sunday to Monday, "compared with 2,240 explosions in the previous 24 hours." The escalation is taking place just days after President Trump and Russian President Vladimir Putin had their first conversation as heads of state.

Russia accused the Ukrainians of starting the latest round of fighting. "Instead of efforts to achieve sustainable peace, the Ukrainian authorities are trying hard to achieve a military solution to the conflict," the Russian foreign ministry said in a statement Tuesday. "Strangely enough, every escalation of the situation in Donbass comes at a time when the Ukrainian leadership is away on a foreign trip. Clearly, this is an attempt to keep the crisis provoked by Kiev on the international agenda."

The city is strategically significant for a variety of reasons, including proximity to rebel-held territory. "Just as important is the town's coking and chemical plant — the biggest of its kind in Europe," the BBC explained. "If it fell into rebel hands it would cut much-needed supplies to Ukraine's steel industry."

City officials are planning to evacuate the residents, as the fighting has cut off public utilities such as "water, electricity, or heat," according to Byrnes. "Ukraine has called for an immediate period of silence to allow repairs to critical infrastructure," she said. "It is also attempting to convene an emergency meeting of the Trilateral Contact Group and has urged the Special Monitoring Mission to step up patrols in the area. Nonetheless, combined Russian-separatist forces have insisted they will continue to fight until Ukrainian forces retreat."