* Ukraine's Monday ultimatum to separatists saps risk appetite
* Japan's Nikkei touches fresh 6-month low
* Yen, gold bolstered as investors seek safety
* Euro undermined by ECB officials' talk of more easing
Asian share markets gave up more ground on Monday after a dismal week on Wall Street and tensions in Ukraine sapped investors' appetite for risk, which helped underpin the safe-haven yen.
Ukraine gave pro-Russian separatists a Monday morning deadline to disarm or face a "full-scale anti-terrorist operation" by its armed forces, raising the risk of a military confrontation with Moscow.
European Union foreign ministers will hold talks later on Monday about tougher sanctions against Russia.
MSCI's broadest index of Asia-Pacific shares outside Japan shed 0.4 percent, pulling further away from five-month highs hit on Thursday.
Japan's Nikkei stock average briefly turned positive, but was down about 0.1 percent after hitting its lowest level since early October. The Nikkei tumbled 7.3 percent last week, its biggest weekly fall since the devastating earthquake and tsunami in March 2011.
Investors were wary that Wall Street's rout might continue.
"Some are worried that a U.S. bubble in equities markets might be corrected, because of the ongoing tapering" of monetary stimulus by the U.S. Federal Reserve, said Kyoya Okazawa, head of global equities at BNP Paribas in Tokyo.
S&P 500 e-mini futures were down about 0.2 percent on Monday, after U.S. stocks slid in a volatile session on Friday. The Nasdaq closed below the 4,000 mark for the first time since early February as investors bailed out of high-flying technology and biotech shares.
The low-yielding yen benefited from the heightened risk aversion. The dollar was down about 0.1 percent at 101.55 yen , after touching a 3-1/2-week low of 101.32 yen on Friday, a far cry from a 2-1/2-month high of 104.13 yen set on April 4.
"Wall Street's performance will remain a key driver for the dollar and yen. Near-term focus is on 101.20 yen. It appears significant bids for the dollar are lined up there, and a break below that level is likely to trigger significant covering of yen shorts," said Junichi Ishikawa, market analyst at IG Securities in Tokyo.
A break below 101.20 yen, a low hit on March 3 when Russia was tightening its grip on Crimea, would take the dollar to a 10-week nadir.
The dollar index steadied, rising about 0.2 percent to 79.604, although April 4's seven-week high of 80.599 remained a distant memory after the greenback's battering last week as U.S. stocks tumbled.
The dollar got some help against the euro from European Central Bank officials, whose comments rekindled speculation about more easing in the euro zone.
The euro fell about 0.3 percent to 140.69 yen. Against the dollar, it shed about 0.2 percent to $1.3851, moving away from a 3-1/2-week peak of $1.3906 hit on Friday.
ECB President Mario Draghi on Saturday told a news conference that "a further strengthening of the exchange rate would require further stimulus."
The ECB is ready to make asset purchases if it deems them necessary to counter a prolonged period of low inflation, ECB Executive Board member Benoit Coeure said on Sunday. ECB governing council member Christian Noyer said on Monday in an interview with daily newspaper Le Figaro that euro weakening was desirable.
Spot gold XAU= benefited from the move towards safe-haven assets, adding about 0.6 percent to $1,326.21 an ounce, after earlier marking a new three-week high.
U.S. crude for May delivery added 0.5 percent to $104.28 per barrel, bolstered by fears that the Ukraine situation could escalate. Ukraine is a major supply route for Russian gas to Europe.