(Reuters) - U.N. Secretary-General Antonio Guterres on Tuesday called for a four-day Orthodox Easter humanitarian pause in fighting in Ukraine to allow for the safe passage of civilians to leave areas of conflict and the delivery of humanitarian aid to hard-hit areas.

       Guterres said the United Nations was submitting detailed plans to the parties and was ready to send humanitarian aid convoys to Mariupol, Kherson, Donetsk and Luhansk beginning on Holy Thursday and running through Sunday, the date of Orthodox Easter, which is celebrated by most Ukrainians and Russians.

       "Humanitarian needs are dire. People do not have food, water, supplies to treat the sick or wounded or simply to live day-to-day," Guterres told reporters at the United Nations in New York.

       Moscow's invasion of Ukraine, the biggest attack on a European state since 1945, has killed or wounded thousands. More than 12 million people need humanitarian assistance in the country today, Guterres said.

       Humanitarian ceasefires between Ukrainian and Russian forces in Ukraine are not on the horizon right now, but may be possible in a couple of weeks, the U.N. aid chief, Martin Griffiths, said on Monday.

       U.N. aid officials are planning to dispatch a humanitarian convoy in the next couple of days into the eastern Donetsk region, where Russia-backed separatists declared a republic, and from there aid supplies would go to Luhansk, another separatist region, he said.

       "Instead of celebration of a new life, this Easter coincides with a Russian offensive in eastern Ukraine. The intense concentration of forces and firepower makes this battle inevitably more violent, bloody and destructive," Guterres said.

       "The onslaught and terrible toll on civilians we have seen so far could pale in comparison to the horror that lies ahead."

       (Reporting by Daphne Psaledakis and Jonathan Landay in Washington; Editing by Alistair Bell)