Ukraine will block a convoy of 280 Russian trucks reportedly filled with food, water and other supplies because it believes the convoy is a covert military operation disguised as humanitarian assistance.

Russian news agencies reported that hundreds of white trucks were being packed with supplies and sent to the eastern Ukraine border, but a spokesman for Ukraine's National Security and Defense Council, Andriy Lysenko, said Ukraine would not allow the trucks to cross into the country because the aid was not certified by the Red Cross.

"This convoy is not a certified convoy. It is not certified by the International Committee of the Red Cross," Lysenko said, according to the Associated Press.

Another Ukraine presidential aide, Valery Chaly, told reporters that the supplies would be loaded onto other transport vehicles before they crossed into Ukraine territory held by separatists, Reuters reported.

Western countries are worried that Russian President Vladimir Putin will use the supposedly humanitarian aid as a pretext for invading Ukraine after he justified Russia's initial invasion of Crimea on similar grounds.

More than 1,000 people have died and thousands more are believed to be running out of water and medical supplies because of the fighting in Donetsk and in the border town of Luhansk, according to Reuters.