MOSCOW : A Russian convoy of 280 trucks carrying "humanitarian supplies" was headed to conflict-torn east Ukraine on Tuesday, despite warnings from the West that Moscow could be using the mission as a guise to sneak in troops.

Russian President Vladimir Putin had on Monday justified the mission citing "catastrophic consequences" of Ukraine's offensive against insurgents in eastern Ukraine, and said Moscow was working in collaboration with the Red Cross.

But the international aid agency said on Tuesday there was still no agreement on the issue and France insisted such convoys should not be allowed to cross the border unless they met strict conditions, including Red Cross approval. Suspicions about Russia's intentions are running high in the West which has accused Moscow of fanning the insurgency in eastern Ukraine by supplying pro-Russian separatists with arms.

Russia firmly denies the allegations but NATO says Moscow has massed 20,000 troops along the Ukrainian border. Kiev has put the number at 45,000.

As wrangling continued over potential aid delivery, besieged rebel-held cities warned of a looming humanitarian disaster as fierce fighting left residents without power, running water or fuel.

Medical and food supplies were also running low in the separatist bastions of Donetsk and Lugansk - both of which have been rocked by intense shelling in recent days.

Ukrainian forces have surrounded Donetsk, a rebel stronghold with a population of about one million, and the army said on Tuesday it was now ready to encircle Lugansk, another bastion with about 500,000 inhabitants.

If Kiev's forces consolidate their gains, they could cut off rebel access to the porous borders with Russia - from where Ukraine believes the weapons are being supplied.

As the fighting intensifies, Moscow has been pushing for such a humanitarian mission, and has even asked the West to back its plans.

But Kiev said at the weekend it had halted a Russian convoy accompanied by troops, although it gave no details.

Its claim sparked strong reactions from the West, with US President Barack Obama telling Moscow any further attempts would be "illegal".

As the Russian trucks carrying 2,000 tons of humanitarian supplies, including medical equipment, baby food and sleeping bags, headed to Ukraine, France reiterated the West's fears of a Russian trojan horse.

"We have to be extremely careful because this could be a cover by the Russians to set themselves up near Donetsk and Lugansk and declare a fait accompli," Foreign Minister Laurent Fabius said.

He also laid out strict conditions for any aid mission, saying it "is only possible and only justifiable if the Red Cross gives its consent, if there are no military forces around (the mission), if there are not just Russians but other countries and if Ukraine agrees." "At this precise moment, this is not the case," he said.

The International Committee of the Red Cross confirmed that no green light had been given for an aid mission.

"We still need to get some more information before we can move ahead," ICRC spokeswoman Anastasia Isyuk said in Geneva.

Former Ukrainian president Leonid Kuchma, who is representing Kiev in talks with Moscow and European security monitor OSCE, told Interfax Ukraine news agency that parties agreed that any aid should pass through government-held Kharkiv region before heading to Lugansk.

Kuchma added that any mission would only be carried out under the auspices of the Red Cross and that monitors from OSCE would accompany the convoy.

Over 1,300 people have been killed in four months of what the Red Cross has already deemed a civil war, while 285,000 have fled their homes according to the United Nations.