But sluggish growth in developed countries and added drag of debt crises in the eurozone and the United States, pose a threat to the health of funds built up by people contributing to, or drawing from, private pension schemes.

Conditions on sovereign debt markets, and also stock markets, are critically important to pension funds, as to most other sectors of the investment fund industry, because most of their savers' funds is invested in bonds and stocks.

"Having weathered the financial crisis, pension fund asset levels in most countries continued to show strong growth throughout 2010," the Organisation for Economic Cooperation and Development said.

But the outlook for future economic growth in developed economies is "uncertain and sluggish," and pension funds could suffer in the medium term, the OECD said warning that a fall in interest rates would weaken overall fund performance.

In most of the 34 OECD member countries, private pension schemes are an alternative, or an addition to, obligatory basic state pension plans.

Private funds invest the money saved by contributors mainly in sovereign bonds for safety and a steady income and in shares, but also in other assets such as property and to some extent in higher risk sectors such as hedging and derivative instruments

The size of funds can be huge in relation to the economy as a whole and private pensions are major players on world financial markets.

For example, at the end of last year private funds in the Netherlands were worth the equivalent of about twice what the entire Dutch economy produces in a year and in Britain they were worth 86.6 percent of annual national production. On average, in OECD countries, the total value of assets held by pension funds amounts to 79.1 percent of national gross domestic product.

---Pensions hit hard in Greece and Portugal---

Last year the net return on these investments OECD-wide was 2.7 percent, down from 4.3 percent in 2009, but performance in terms of return on investment varied widely.

Private pension funds in New Zealand outperformed with a return of 10.3 percent and in Poland they achieved a net return of 7.7 percent.

But the return for funds in Greece and Portugal, both hard hit by national debt crises and consequent corrective budget cuts, was heavily negative.

Greek funds generated a negative return of 7.4 percent and Portuguese funds 8.1 percent.

The negative figure for Greece was "due to the collapse of the Athens stock exchange as well as the drop in price of Greek bonds."

In Portugal the explanation lay in "adverse capital market performance".

Pension funds in the OECD invest more than half of all the money flowing in from contributions into government debt bonds, which represent a specific so-called "asset class", usually referring to instruments with a strong credit rating and therefore representing relatively low risk.

This suggested "a conservative stance", the OECD said, as bonds provide a steady and predictable revenue stream.

The greatest losses incurred during the worst of the economic crisis were the result of heavy falls in shares prices, the OECD said.

The OECD said private pension fund schemes were becoming based increasingly on pre-defined contribution levels.

This is as opposed to pensions tied to the salary earned in the last few years of employment irrespective of how the assets have performed.

Increasingly employers were closing such final salary schemes, the OECD said.

Copyright AFP (Agence France-Presse), 2011