Analysts were watching warily to see whether the impact of intervention will be lasting, cautioning that the maximum benefit from it likely would occur on Friday. U.S. officials were maintaining a stoic silence about any further actions.
"Usually with each intervention, you end up seeing a diminishing marginal return. So the first-time round usually has the biggest impact," said David Morgan, head of research for the Americas at Standard Chartered in New York.
"It'll become increasingly difficult for intervention to be successful," he added.
U.S. Treasury Secretary Timothy Geithner, who heads for Brazil on Friday night with President Barack Obama, spoke with French Finance Minister Christine Lagarde, whose government currently chairs the G7, and Japanese Finance Minister Yoshihiko Noda ahead of Thursday night's intervention call.
Geithner let Obama know ahead that the G7 was embarking on a coordinated intervention.
The United States has been at pains to stress its willingness to help stricken Japan overcome the triple disaster of the earthquake, tsunami and nuclear crisis.
In a measure of the severity of the situation Tokyo now faces, Japanese engineers conceded on Friday that burying a crippled nuclear plant in sand and concrete may be a last resort to prevent a catastrophic radiation release.
That was the method used to seal huge leakages from Chernobyl in 1986 and was taken as a sign that Japan's piecemeal actions, such as dumping water from military helicopters or scrambling to restart cooling pumps, may not work.
(Additional reporting by Wayne Cole, Wanfeng Zhou, and Daniel Flynn; writing by Glenn Somerville; Editing by Leslie Adler)