European insurance and reinsurance shares were under renewed pressure Monday on mounting concern over the safety of Japan's nuclear power plants and following weekend claims estimates suggesting Friday's devastating earthquake could be the sector's costliest ever.

At 0900 GMT, Major players such as German reinsurer Hannover Re (HNR1.XE) were 7.5% lower at EUR35.70, while Munich Re (MUV2.XE) fell 4.3% to EUR106.80. German insurer Allianz SE (ALV.XE) was down 3.2% at EUR96.43, and Swiss Reinsurance Co. (RUKN.VX) fell 3.2% to CHF50.05.

Allianz, Hannover Re and France's Scor SE (SCR.FR) reiterated previous comments that it was too early to come up with loss estimates. Scor was unable to say immediately if it had exposure to Japan's nuclear plants.

Munich Re said in a statement Monday that while it was too early to calculate potential losses that the insurance sector was unlikely to be significantly hit by problems tied to Japan's nuclear plants. Industry experts also said that the Japanese government would likely bear the bulk of nuclear-related costs.

Chaucer Holdings PLC (CHU.LN), a Lloyd's insurer, was one of the few insurance shares to make gains. It was up 0.5% at 54 pence after saying Monday it doesn't expect any significant insured losses from the Japanese earthquake. It said that its specialist Nuclear Syndicate 1176 is one of a panel of insurers that provide coverage to Tokyo Electric Power Company, the owner of two of the three nuclear sites in the proximity of the affected area--Fukushima Dai-ichi and Fukushima Daini--but that there was no coverage in place for property damage or business interruption at these two plants. At a third plant, Onagawa, owned by Tohuku Electric Power Company, coverage for property damage is provided, but earthquakes and tsunami are specifically excluded, it said.

UBS analysts said Monday they expect Hannover Re's losses to be capped by its own reinsurance agreements, even though recent natural disasters have already absorbed 50% of the company's 2011 major loss budget.

While the exact cost of the catastrophe won't be known for months, Boston-based AIR Worldwide estimated Sunday the quake caused insured property losses of $15 billion to $35 billion. If claims come in at the middle of that range, the cost of the disaster would surpass all other natural catastrophes except for 2005's Hurricane Katrina.

Nomura analysts estimated that Munich Re, Swiss Re and Hannover Re could see around 3% to 6% of their capital absorbed by losses, or between $300 million to $2.8 billion. "We are likely to see material hits on earnings, but not capital events."

The area of the country most directly affected by the quake has about $300 billion of insured property, according to AIR Worldwide. An additional $400 billion of insured property lies in areas that were less impacted but likely still sustained costly damage, said Jayanta Guin, senior vice president of research and modeling at Boston-based AIR.

-By William Launder; Dow Jones Newswires; +49 69 29 725 515; william.launder@dowjones.com