Ukraine's military claimed hundreds of Russian troops crossed the border to help the rebels, as intense fighting between government and pro-Russian separatist forces continued Monday in eastern Ukraine.
In Kyiv on Monday, Ukrainian Prime Minister Arseniy Yatsenyuk said Ukrainian intelligence had confirmed Russian cross-border arms deliveries to the separatists were continuing.
“Tanks, howitzers, Grad systems, Smerch, Buk,” Yatsenyuk told a joint news conference with Polish Prime Minister Ewa Kopacz, listing Russian-made missile systems that he said were being channeled to the separatists.
“Radio-electronic surveillance stations are not on sale in the Donetsk market -- they are only to be had from the Russian defense ministry and Russian military intelligence,” he added.
Ukrainian military spokesman Andriy Lysenko said about 700 Russian troops had also crossed into Ukraine to back up the rebel forces.
"This morning, two groups of armed forces from the Russian Federation crossed the border," Lysenko told the French news agency AFP.
Fighting near airport
After a brief pause in fighting early Monday as Orthodox Christians celebrated Epiphany, clashes reignited in the afternoon, with regular explosions heard coming from the direction of the Donetsk airport, AFP reported.
The general staff of Ukraine's armed forces claimed they had established control over the disputed Donetsk airport after driving out rebel forces, who it said had retreated to residential areas of the city of Donetsk.
Although a rebel spokesman said Sunday night that the separatists had repelled an attack by government forces and remained in control of the shelled facility.
Ukrainian officials said three soldiers had been killed and 66 wounded over the past 24 hours, during which they said they had returned battle lines at the airport outside Donetsk to the status quo under the Minsk-brokered, 12-point peace plan.
Both government and separatist forces have been accused of repeatedly violating the truce.
In other violence Monday, a blast outside a courthouse in the eastern Ukrainian city of Kharkiv wounded at least 12 people.
The explosion came just after supporters of the Ukrainian nationalist Svoboda party left the building, where a party member is on trial.
A top prosecutor says it is too early to say if Svoboda was involved, but he says the blast will be investigated as a terrorist act.
Meanwhile, Russia, expressing concern at what it called an escalation by Kyiv, published its own peace plan on Monday in the form of a letter from President Vladimir Putin to Ukraine's President Petro Poroshenko, which it said Poroshenko had rejected.
“It's the biggest, even strategic mistake of the Ukrainian authorities to bank on a military solution to the crisis,” Interfax quoted Deputy Foreign Minister Grigory Karasin as saying. “This may lead to irreversible consequences for Ukrainian statehood.”
Concerns mounted over the fate of civilians caught in the fighting.
The Donetsk city council reported Monday that eight people had been killed and 29 injured in the city since Friday.
Separatist officials and witnesses told Reuters that a hospital in the center of the city had been shelled. Rebel officials said a surgeon and five elderly patients were injured in the incident, which they blamed on government forces.
The warring sides in eastern Ukraine regularly accuse each other of deliberately targeting civilian areas.
Russia is under heavy sanctions from the West over its alleged actions in Ukraine, and the United Nations Security Council is due to discuss the worsening crisis at a meeting on Wednesday.
The World Health Organization said more than 4,800 people have been killed in the conflict since last April.
Some material for this report came from Reuters and AFP.