Russia has assembled about 20,000 soldiers on Ukraine's border and could use the pretext of a humanitarian mission to invade, NATO has said, its starkest warning yet that Moscow could soon mount a ground assault against its neighbour.
Kiev's military offensive has pushed the rebels out of many of their strongholds, leaving them largely besieged in the cities of Donetsk and Luhansk, which the rebels have proclaimed capitals of two "people's republics".
Residents in Donetsk, east Ukraine's main industrial hub and now the principal rebel redoubt, said Ukrainian warplanes had carried out air strikes overnight.
"Russia has amassed around 20,000 combat-ready troops on Ukraine's eastern border," NATO spokeswoman Oana Lungescu said in a statement on Wednesday.
"We're not going to guess what's on Russia's mind, but we can see what Russia is doing on the ground - and that is of great concern."
Moscow could use "the pretext of a humanitarian or peacekeeping mission as an excuse to send troops into Eastern Ukraine", Lungescu said.
Speaking to Al Jazeera, Pavel Felgenhauer, a military analyst, said: "It is possible that this could happen, it is not the first time such an accusation has been made, but it does not seem like a definite decision has been made.
"It is certainly something Russia could do, but it depends how the situation on the ground develops as to whether they will."
Russian Defence Minister General Sergei Shoigu said the country's peacekeeping forces should be in constant readiness to perform their tasks, Moscow's Interfax news agency reported.
"The world has changed, has changed dramatically. As you know from previous examples, including the brigade, peacekeeping units can be required suddenly," Shoigu said on Wednesday.
"That is why the division and brigade of peacekeeping forces should be in constant combat readiness," the minister added.
However, Russia on Wednesday denied US and NATO claims that it had increased the number of troops along the border with Ukraine.
"I would like to explain to Pentagon and NATO officials that movements of such forces of thousands of troops and equipment is not possible in such a short time, moreover without OSCE (Organisation for Security and Co-operation in Europe) monitors stationed in the region noticing," Russian defence ministry spokesman Igor Konashenikov was quoted as saying by Interfax.
Humanitarian situation worsening
In an emergency meeting held on Tuesday, a senior UN official warned that the ongoing violence in Ukraine will affect four million people living in the country's east if a political solution is not reached.
John Ging, director of UN humanitarian operations, told the Security Council that the humanitarian situation in eastern Ukraine is steadily worsening as power and water supplies become scarce, homes are destroyed and health workers flee.
In Donetsk and Luhansk, the water supply has been cut to a few hours per day, health supplies are running low and an estimated 70 percent of health personnel have fled, he said.
Vitaly Churkin, Russia's UN ambassador, said the situation in the east, particularly in Luhansk and Donetsk, is "disastrous". He accused the Ukrainian military of indiscriminate shelling of housing.
In many small towns, he said, 80 percent of the houses have been destroyed and hundreds of buildings have collapsed.
Churkin said Russia wants to send a humanitarian convoy to Luhansk and Donetsk under the auspices of the International Committee of the Red Cross.
To date, 1,367 people, civilians and combatants, have been killed, according to the UN.