DONETSK, Ukraine — As many as 500 pro-Russia insurgents assaulted a border command center in the eastern Ukrainian city of Luhansk before dawn Monday, sparking a furious battle that raged throughout the day and into the night.
The attack, by rebels armed with mortars and rocket-propelled grenades, was perhaps the largest and most coordinated assault on a Ukrainian government building in the nearly two months that separatists have waged their insurgency here.
Border guards killed at least five rebels in repelling the attack on the base. At least 10 border service officers were injured.
The scale of the fight reflected the critical importance of the border to both sides. In recent weeks, it has frequently been infiltrated by separatists bringing reinforcements and supplies from Russia to eastern Ukraine. The shipments have helped transform the insurgency from a somewhat ragtag guerrilla force to one capable of carrying out major military assaults. That transformation has struck an ominous note for Ukraine’s prospects of hanging on to its southeastern provinces.
Lieutenant Colonel Vladislav Seleznev, a Ukrainian military spokesman, said late Monday that the border guards, with help from Ukrainian special forces and the air force, had fended off the assault. But it was unclear whether the rebel contingent had been defeated or was simply regrouping for another push.
Ukrainian officials fear that a defeat at the command center in the eastern region of Luhansk will open the gates to a flood of new militants. ‘‘If they capture that building, they control all the posts along the border’’ in Luhansk, said Oleg Slobodyan, a spokesman for Ukraine’s border patrol.
Monday’s battle again focused attention on Russia’s role, with US officials accusing Moscow of helping to orchestrate the violence.
‘‘It’s a further example of the destabilizing activities supported by Russia in the east, and it’s something we strongly condemn,’’ said Derek Chollet, assistant secretary of defense for international security affairs, who was in Kiev meeting with senior Ukrainian officials.
After months of blocking any Security Council action on Ukraine, Russia called an emergency meeting of the Security Council on Monday to introduce a resolution calling for an immediate halt to the clashes in the east, the Associated Press reported.
Foreign Minister Sergey Lavrov of Russia said Moscow will seek Security Council action to end weeks of violence between government troops and pro-Russia insurgents.
Lavrov said the resolution would seek to establish a ‘‘stable and reliable cease fire’’ and also call for the creation of ‘‘humanitarian corridors that will help civilians leave hostility zones, should they wish to do so.’’
US State Department spokeswoman Jen Psaki countered that Russia was being ‘‘hypocritical’’ by calling for a cease-fire and seeking help for civilians to safely leave combat zones in eastern Ukraine while ‘‘doing nothing to stop’’ Ukrainian separatists from attacking targets in the east and holding international monitoring teams hostage.
‘‘So if they’re going to call for . . . reduction in tension and a deescalation, it would be more effective for them to end those activities,’’ Psaki said.
The United States has provided $18 million in nonlethal military assistance to Ukraine, including meals-ready-to-eat, basic field supplies, and tents. The United States is interested in providing more assistance to the border service and the military, but any increase in aid risks further antagonizing Russia.
Lavrov told reporters in Moscow that Ukraine’s government was targeting innocent people. ‘‘People die every day and civilians suffer increasingly. The army, combat aviation, and heavy weapons continue to be used against them,’’ Lavrov said, according to Russia’s Interfax news agency.