Military depots, warehouses and National Guard were hit with artillery blasts, the ministry said.As dawn broke in Kyiv, President Volodymyr Zelensky of Ukraine said that he had declared martial law. The country’s defense minister told citizens that the army was “fending off enemy forces” and “doing everything it can to protect you.”But the army was under siege. In the east, Russia-backed separatists — their ranks bolstered by the arrival of hundreds of Russian mercenaries in recent days, according to European officials — said they were hammering Ukrainian troops along the entire 250-mile front line that has divided the rebels and Ukrainian forces since 2014.Seeking to capture the entire territories of Donetsk and Luhansk, which Mr. Putin recognized as independent on Monday, the rebels were “using all weapons at their disposal,” the Russian news media reported. Ukrainian officials said the attacks included artillery strikes.Ukraine’s state border service reported that Russian troops stationed in Belarus, north of Ukraine, had launched an attack with support from the Belarusian military. Russia had deployed as many as 30,000 troops to Belarus for exercises this month that the United States warned could provide cover for an attack against Kyiv, which lies a fast 140-mile drive away from a main border crossing. President Aleksandr G. Lukashenko of Belarus denied that his forces were involved.By midmorning in Kyiv, Russia’s Defense Ministry said it had disabled all of Ukraine’s air defenses and air bases. Ukraine’s Interior Ministry said that Russian forces had captured two villages in the Luhansk region.The fighting intensified on Thursday afternoon.Ukrainian forces shot down six Russian fighters and a helicopter in a fight to maintain control over key cities, a senior Ukrainian military official said, speaking on the condition of anonymity to release information outside official channels. Ukraine’s defense minister, Oleksiy Reznikov, called on all Ukrainian civilians to join the fight and enlist with territorial defense units.“Ukraine is moving into all-out defense mode,” he said.