By Suleiman al-Khalidi

AMMAN, Jan 22 (Reuters) - The U.N. is launching a major aid effort to help tens of thousands of children suffering in the aftermath of Israel's military campaign in Gaza, a top aid official said on Thursday.

"We are scaling up our humanitarian support and looking to expand significantly our psychosocial support. The situation is very dire," Sigrid Kaag, UNICEF Regional Director for the Middle East and North Africa, told Reuters in Amman.

Kaag said children, who form the majority of the 1.5 million Palestinians in 360-sq km (139-sq mile) Gaza Strip, bore the brunt of the three-week onslaught by Israeli troops to counter rocket attacks from the territory.

More than 400 children were among the 1,314 Palestinians killed and more than 1,800 children were injured, many suffering amputations or severe burns, the U.N. official said.

"We are dealing with families that have suffered immeasurable losses," Kaag said. "The state of injury of children is very worrisome, from moderate to severe ... which often of course impacts the chances of rehabilitation."

UNICEF aims to offer mass counselling and first aid. "We are expanding and expanding and when we get the centres up and running we will very quickly reach 200,000 persons," said Kaag.

"The Gaza Strip is an enclosed environment. There was no safe haven, there was no shelter and there was no place to flee and children always in these conditions end up being more vulnerable," she said. At least 50,000 are still in shelters.

"Our first infrastructure priority is to fix the schools quickly."

U.N. technical teams are aiming to open schools by mid-February and to repair quickly key water supply systems damaged by the intensive bombing.

But much of the international aid planned for post-war Gaza depends on Israel allowing it to flow through border crossings it controls.

"Obviously full unhindered access is necessary. It is evolving on a daily basis and it is in the interest of everybody to see Gaza be able to restore itself," Kaag said.

"The full process of rehabilitation and reconstruction of what was in the Gaza Strip ... is a process of a number of years."

(Writing by Suleiman al-Khalidi; editing by Andrew Roche)