* Asian stock markets : tmsnrt.rs/2zpUAr4
* S&P 500 futures fall again in Asia, Nikkei at 15-mth low
* Chinese shares retreat amid U.S. accusations of hacking
* Trump threatens partial U.S. govt shutdown over wall funding
* Dollar nurses losses on yen as investors flee to safe havens
* Oil near lowest in over a year, an added bonus to bonds
By Wayne Cole
SYDNEY, Dec 21 (Reuters) - Global stocks extended a steep sell-off on Friday as the threat of a U.S. government shutdown and of further hikes in U.S. borrowing costs sent dismayed investors scurrying for safer assets.
Financial spreadbetters pointed to a modestly lower start in Europe, with London's FTSE likely to drop 0.3 percent, Paris' CAC down 0.2 percent and Frankfurt's DAX off half a percent.
The S&P 500 was heading for its worst quarter since the dark days of late 2008, with a loss of 15 percent so far. The Nasdaq has shed 19.5 percent from its August peak, just shy of confirming a bear market.
Oil prices slid just over 4 percent overnight, bringing Brent's losses since its October top to 37 percent. The dollar had suffered its biggest one-day drop on the yen since November 2017 as investors stampeded to safe havens.
Michael McCarthy, chief market strategist at CMC Markets, said the downward spiral was becoming self fulfilling with selling begetting more selling.
"Negative momentum is a key factor in driving investor behaviour. Fundamental justifications are following the action," said McCarthy. "The selling will finish when it is done."
E-Mini futures for the S&P 500 were off another 0.1 percent, while MSCI's broadest index of Asia-Pacific shares outside Japan dropped 0.2 percent.
Japan's Nikkei lost 1.1 percent to close at its lowest since mid-September last year, after giving up 5.6 percent this week. Australian stocks slipped 0.7 percent, hovering just above a two-year trough hit earlier in the session.
Chinese blue chips lost 1.4 percent, in part after the United States accused Beijing of orchestrating the hacking of government agencies and companies around the world.
Sentiment had turned sour on Thursday when the U.S. Federal Reserve largely retained plans to increase interest rates despite mounting risks to growth.
Markets were further spooked when U.S. President Donald Trump refused to sign legislation to fund the U.S. government unless he got money for a border wall, thus risking a partial federal shutdown on Saturday.
"Political brinkmanship in Washington is further heightening market uncertainty," said Westpac economist Elliot Clarke.
"Friday will be a tense day in Washington, and for financial markets, as a last-minute compromise is sought."
Adding to the air of crisis was news U.S. Defense Secretary Jim Mattis had resigned after Trump proposed withdrawing troops from Syria and sources said a military pullback from Afghanistan was on the cards.
The brittle mood showed on Wall Street where the Dow ended Thursday with a loss of 1.99 percent. The S&P 500 dived 1.58 percent and the Nasdaq 1.63 percent.
The mood change has triggered a rush out of crowded trades, including massive long positions in U.S. equities and the dollar and short positions in Treasuries.
Lipper data on Thursday showed investors pulled nearly $34.6 billion out of stock funds in the latest week and were heading for the biggest month of net withdrawals on record.
There was also a sense of capitulation in currency markets as the dollar dived 1.1 percent on the yen on Thursday to hit a three-month trough at 110.80. It was last changing hands at 111.41 having shattered several layers of chart support.
The euro stood at $1.1454, having jumped to its highest in over six weeks at $1.1485. Against a basket of currencies, the dollar stood at 96.399 after suffering its largest single-session fall in two months.
The flight from risk was a boon to sovereign bonds, where U.S. 10-year yields struck their lowest since early April at 2.748 percent. As recently as October, they had been at a seven-year top of 3.261 percent.
The gap between two- and 10-year yields shrank to just 9 basis points at one stage, the kind of flattening that has heralded recessions in the past.
The rally in longer-dated bonds has been fuelled by the huge slide in oil prices, which will pile downward pressure on inflation at a time when the global economy is already slowing.
Both Brent and U.S. crude futures reached their lowest in more than a year overnight, but edged higher on Friday on talk production cuts by OPEC might be larger than first thought.
U.S. crude eked out a 34 cent bounce to $46.22 a barrel, while Brent rose 32 cents to $54.67.
Gold was underpinned by the reversal in the dollar to hold at $1,259.00 an ounce.
Additional reporting by Daniel Leussink; editing by Jacqueline Wong & Shri Navaratnam