TOKYO, Sept 7 (Reuters) - Foreign investors were big buyers of Japanese bonds last week, with geopolitical tensions and concerns over U.S. fiscal policy boosting demand for such safe (if low-yielding) assets.
Weekly data from Japan's finance ministry issued on Thursday showed foreign investors bought a net 1.3592 trillion yen ($12.46 billion) of medium- to long-dated Japanese bonds in the week through Sept. 2.
This net purchase was the third largest in records stretching back to 2005.
In what seems to be a paradox for some, Japan has served as a safe-haven of sorts with the yen often gaining in times of global risk aversion.
Japan is the world's biggest creditor nation and there is an assumption that investors there will repatriate offshore funds in a crisis, boosting demand for yen.
Demand for safer assets such as government debt were in high demand last week after North Korea's ballistic missile launch alarmed global markets.
The yield on the Japanese government two-year bond (JGB) fell to a three-month low of minus 0.170 percent on Aug. 31 as bond prices rose. It last stood at minus 0.155 percent.
JGB yields have been kept near historical lows under the Bank of Japan's ultra-loose monetary policy.
The benchmark 10-year JGB yield declined to a 10-month trough of minus 0.010 percent on Monday and last stood at 0.015 percent.
"We saw debt yields decline broadly last week on heightened geopolitical risks and concerns over the U.S. fiscal situation. And buying of Japanese bonds by foreigners stood out, as domestic investors are not keen on buying bonds that yield below zero," said Noriatsu Tanji, senior bond strategist at Mizuho Securities.
Investors have been worried whether the U.S. Congress would fail to raise the country's debt ceiling and face a technical default, leaving the Treasury unable to issue debt.
Foreign investors were seen to have bought short-term Japanese bills as an alternative to U.S. Treasury bills because of their fears of a potential default.
In turn, demand that could not be fully met at the short end spilled over to longer-dated maturities.
"The short end of the yield curve is firmly anchored, so downward pressure on the curve spreads easily through the two-year, five-year and 10-year maturities," said a trader at domestic bank.
Still, observers said the latest bout of foreign investor buying needed to be seen in context.
"Monthly data shows that unlike last year, foreign investors' buying of Japanese bonds has been sluggish so far this year. Last week's spike in buying came after a period of relatively thin flows," said Tanji at Mizuho Securities.
According to finance ministry data, foreigners bought a net 4.747 trillion yen ($43.51 billion) of mid- to long-term Japanese bonds in the first six months of 2016, but had only purchased a net 1.951 trillion yen during the same period in 2017.
Foreign investors sold a net 158.3 billion yen of Japanese equities in the week through Sept. 2.
($1 = 109.1100 yen)
(Reporting by Shinichi Saoshiro; Additional reporting by Hiroyasu Hoshi in Tokyo; Editing by Eric Meijer)