John Ging, director of UN humanitarian operations, said at an emergency meeting on Tuesday that violence, especially in urban areas, will put more people at risk and lead to "an increase in the numbers killed" if a political solution cannot be reached.
He warned 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.
"Immediate action is therefore required to prevent this," he told council members.
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.
NATO, meanwhile, said Russia had amassed about 20,000 troops on Ukraine's eastern border and were concerned Moscow could use "the pretext of a humanitarian or peace-keeping mission as an excuse to send troops into Eastern Ukraine," spokeswoman Oana Lungescu said in a statement.
Stating the conflict in Ukraine was fuelled by Russia, Lungescu said that the troop build-up had further escalated "a dangerous situation".
"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. Russia has amassed around 20,000 combat-ready troops on Ukraine's eastern border," she 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."
To date, 1,367 people, civilians and combatants, have been killed, according to the UN.
The meeting, requested by Russia, comes after fighting in eastern Ukraine reached the main rebel stronghold of Donetsk.
Meanwhile, 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.
Pro-Russia separatists and government troops on Tuesday exchanged rocket fire in the latest clashes since the two sides began battling in April.
Ukraine and western countries have accused Russia of providing the pro-Russia separatists with weapons and soldiers, a claim the Russian government has repeatedly denied.
Oleksandr Pavlichenko, Ukraine's deputy ambassador, denied there was a humanitarian crisis in Ukraine but said the humanitarian situation in Donetsk and Luhansk is still serious.
"The situation is manageable by the government of Ukraine, which remains open to cooperation with international partners," he said.
Mark Lyall Grant, Britain's UN ambassador, said: "It is deeply ironic that Russia should call for an emergency meeting of the council to discuss a humanitarian crisis largely of its own creation."
Fighting across eastern Ukraine has forced more than 285,000 people to flee their homes, according to UN figures released on Tuesday.
Source: Al Jazeera