The dollar slipped against the yen Monday but remained within a hair's breadth of the key 100 level after major industrialized nations gave a stamp of approval to a massive Japanese easing program, which has eroded the currency.
Japan officials said that the Group of 20 leading economies accepted that the country's $1.4 trillion stimulus program is aimed at conquering 15 years of deflation rather than at weakening the yen.
In response, the dollar climbed as high as 99.90 yen, within striking distance of a four-year high of 99.95 set on April 11 and the 100 level, where option barriers are said to be lined up. It was last at 99.13 yen, down 0.6 percent.
"The lack of pushback by the G-20 effectively gives the BOJ room to ease further if needed and should keep the yen biased broadly lower," said Omer Esiner, chief market analyst with Commonwealth Foreign Exchange in Washington.
A break of 100 could trigger stop-loss buying, which could take the pair up to 101.45 yen, an April 2009 high that could act as near-term resistance. Reported large option expiries at 100 will keep the currency pinned to that level.
But with the yen not hitting similar highs against other currencies, such as the Canadian and Australian dollars, the Japanese currency's weakness against the dollar could be limited.
"I think if you saw the yen in as weak a state against the crosses it might add a little confirmation, it would make people a little more comfortable in selling the yen now against the dollar and against a lot of currencies," said Robert Lynch, head of currency strategy for the Americas at HSBC in New York.
A sharper slide in the yen would also require more information on flows out of Japan, said Adam Cole, global head of FX strategy at RBC Capital Markets, who sees the yen staying around the 100 mark a month from now.
"To make it (a break of 100) sustainable, you need to see strong evidence that Japanese investors are buying rather than selling overseas assets, or you need to see a shift in hedging behavior ... But at the moment we don't see that from capital flows data."
One-month implied volatility on dollar/yen jumped as high as 14.415, a level not seen in two years, underscoring increased demand for options to protect against yen weakness.
Data on Friday showed currency speculators raised their bets against the yen in the week ended April 16, while lifting positions in favor of the U.S. dollar.
The yen has weakened 23 percent against the dollar since mid-November, when Shinzo Abe, who became Prime Minister in December, promised bold monetary and fiscal expansionary policies during his election campaign.
The euro also remained vulnerable against the dollar on central bank expectations. The single currency last traded down 0.1 percent to $1.3062.
Technical analysts at SEB said that a break below $1.3026 would likely "trigger a new round of selling" in the euro, with the next support then seen at $1.3001.
From a fundamental point of view, the euro has been hamstrung by persistent talk of an interest rate cut by the European Central Bank.
The euro "is moving toward 1.30 this morning because more European Central Bank officials sound like they support the idea of a rate cut or some form of additional easing from the central bank," said Kathy Lien, managing director at BK Asset Management in New York.
That makes this week's euro zone PMI and German IFO reports key, Lien added.
Even the re-election of Italy's president offered little support to the single currency, analysts said.
(Read More: Hollow Victory for Italy as More Turmoil Looms)
"Quite frankly, I don't think removing this uncertainty from the equation will necessarily translate into a euro positive ... as it means that there's yet to be resolution to the hung parliament," said Christopher Vecchio, currency analyst at DailyFX.