AMMAN, June 2 (Reuters) - Iraq's environmental problems - among world's worst - range from a looted nuclear site which needs cleaning up to sabotaged oil pipelines, a U.N. official said on Thursday.

"An improvement is almost impossible in these security conditions. Chemicals are seeping into groundwater and the situation is becoming worse and creating additional health problems," said Pekka Haavisto, Iraq task force chairman at the United Nations Environmental Programme.

"Iraq is the worst case we have assessed and is difficult to compare. After the Balkan War we could immediately intervene for protection, such as the river Danube, but not in Iraq," Haavisto, a former Finnish environment minister, said on a visit to Jordan to meet with Iraqi officials.

Lack of spare parts and Iraq's inability to maintain pollution standards during two previous wars and more than a decade of crushing sanctions have damaged the environment, including the Tigris and Euphrates rivers where most of Iraq's sewage flows untreated.

The situation became worse after the 2003 U.S.-led invasion, in which depleted uranium munitions were used against Iraq for the second time and postwar looting and burning of the once formidable infrastructure caused massive spills and toxic plumes, Haavisto said.

"The bombing and war carried a cost but the looting cost the environment more, such as in the Dora refinery or Tuwaitha nuclear storage," Haavisto said.

"There has not been proper cleanup and only assessment work at some of these sites. Very little has changed and Iraqi teams are in the process of getting in some of these locations."

The U.N. official was referring to the 56 square km (22 sq mile) Tuwaitha complex south of Baghdad where 3,000 barrels that stored nuclear compounds were looted.

In the Dora depot on the edge of Baghdad, 5,000 barrels of chemicals, including tetra ethylene lead, were spilt burnt or stolen, a U.N. survey showed.

Contaminated sites near the water supply also include a 200 square km (77 sq mile) military industrial complex, torched or looted cement factories and fertiliser plants, of which Iraq was one of the world's largest producers, and oil spills.

"Iraq was a modern industrial society in many ways. The chemicals are very risky on its future. The more time passes the more consequences on health," Haavisto said.

He said postwar assessment of the environmental damage was proceeding despite threats to the 1,000 staff of an Iraqi environment ministry, set up as an independent unit after the American invasion.

The field studies will eventually include depleted uranium, a toxic, heavy metal used to make bombs more lethal, of which the United States used an estimated 300 tonnes in 1991 Gulf War and an unknown quantity during the last invasion.