CME Group noted that the yen's rally has gained momentum as dollar-selling options were triggered, with a potential break below 155 possibly attracting further dollar selling given the rising demand throughout the day for options struck at the 155 yen level.

The derivatives marketplace operator said the most actively traded contract on Thursday was the overnight dollar put option with a strike price of 155.0 yen. The dollar-yen pair briefly fell 2.1% to 155.34, its lowest level since August 3.

A move below 155.23 would take the pair under the level seen during the most recent yen-buying intervention, with many market participants viewing 155 as a key yen resistance point, as Japan's April intervention failed to push the currency past this threshold.

Interest in the 155 strike rose after the dollar-yen dropped below 156.25, a level that CME said also attracted strong demand earlier in the session.