While the prospect of lower borrowing costs provided support to equity markets, investors trod nervously on concerns about the world's top economy.
Also, Donald Trump dented hopes for a quick resolution to the China-US trade talks by warning tariffs would stay in place for some time after any agreement is reached.
After a much-anticipated meeting the US central bank forecast that it would not raise rates this year -- a shift from an earlier projection of two -- and cut its annual growth outlook.
"It may be some time before the outlook for jobs and inflation calls clearly for a change in policy," Fed boss Jerome Powell said after the meeting.
The announcement took markets by surprise, with most observers expecting it would tee up at least one rate hike this year, and fuelled concerns about the state of the economy.
The greenback sank against its major peers, except the Brexit-hit pound, while higher-yielding units were also well up. The South African rand piled on more than two percent, Mexico's pesos jumped more than one percent and the Australian dollar jumped one percent.
Most stock markets in Asia rose, with Hong Kong up 0.2 percent and Shanghai 0.5 percent higher. Singapore added 0.2 percent, Seoul climbed 0.9 percent and Manila jumped 0.6 percent. Taipei and Jakarta also rose though there were losses in Sydney and Wellington.
- EU has 'lost patience' -
However, there is some unease across trading floors after Trump's remarks. The president said that if a trade deal is reached between the world's top two economies, US tariffs would remain in place "for a substantial period of time".
"We have to make sure that if we do the deal with China, that China lives by the deal," he added.
The comments dented optimism the two sides would reach an agreement to end a standoff that has scythed global markets last year.
"This could be a major sticking point from the Chinese side," said National Australia Bank's Ray Attrill. "We can but hope it's all part of the 'Art of the Deal', but in the meantime (it) means we can't as yet fully price in a trade deal next month, or later, with supreme confidence."
Traders are also casting a wary eye on the Brexit saga after Prime Minister Theresa May asked for a three-month extension to the March 29 deadline for leaving the EU as she struggled to prevent an economically painful no-deal divorce.
European Council President Donald Tusk said the EU could agree to the request but only if British MPs pass her withdrawal deal, which they have already rejected twice.
"The EU has clearly lost patience with Britain" said Jeffrey Halley, Senior Market Analyst, OANDA.
"The EU appear to be taking matters into their own hands, addressing parliament directly with a stark choice: sign off on the deal tout suite or risk being kicked out on 29 March; or take a multi-year extension, hopefully with a new referendum and/or government."
The pound edged up against the dollar, supported mainly by the Fed announcement, but there are fears of a sharp sell-off in the event of a no-deal Brexit.