In her first public remarks since the Federal Reserve took a surprisingly dovish stance on monetary policy in its March meeting, Yellen reaffirmed an optimistic outlook for the US economy.
But she told the Economic Club of New York that the Fed has to move cautiously even as it expects to slowly tighten monetary policy over the next few years following its December rate hike.
But, despite recent comments by some Fed officials that inflation was already speeding up and suggesting the Fed could raise interest rates again over the next few months, Yellen suggested she was still not convinced.
"It is too early to tell if this recent faster pace will prove durable," she said.
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At the same time, Yellen said the Fed still has to keep an eye on the global slowdown and markets turmoil as it decides policy.
"There is much uncertainty, however, about how smoothly this transition will proceed, and about the policy framework in place to manage any financial disruptions that might accompany it," she said of China.
She also said that the sustained collapse of the market for oil and other key commodities is also keeping a cloud over the global economy, even as cheaper prices benefit consumers and manufacturers.
Another sharp fall in oil prices would have an adverse impact on the world economy, she noted.
Reversing earlier losses, the S&P 500 added 0.4 per cent in early afternoon trade, while the dollar lost about 0.6 per cent at $1.1266 per euro. The dollar also fell against the yen and the British pound.