Federal Reserve Chair Janet Yellen said that the Fed remains on track to raise interest rates this year, with labour markets expected to steadily improve and turmoil abroad unlikely to throw the US economy off track. 

"If the economy evolves as we expect, economic conditions likely would make it appropriate at some point this year to raise the federal funds rate," Janet Yellen said.

She made her comments in testimony prepared for the US House of Representatives Financial Services Committee. 

Her comments affirmed the view of a central bank prepared to gradually raise rates after more than six years at a near-zero level. Labour markets are "not yet consistent with maximum employment," she said. 

"Greece remains difficult. And China continues to grapple with the challenges posed by high debt, weak property markets, and volatile financial conditions," she added. 

But "looking forward, prospects are favourable for further improvement in the US labor market and the economy more broadly," she stated. 

Her written statement to the committee is to be followed by a hearing later today. 

The statement largely tracked her recent public comments, as well as the most recent policy statement by the Fed's policy-setting committee. 

She did, however, include an explicit defence of the Fed's "transparency and accountability," detailing the central bank's flow of information to financial markets and its press conference and audit schedules as evidence it does not need further congressional oversight.

She will likely be questioned on that very point from members of the Republican-led House committee. House members were critical of the Fed at her previous appearance before them in February. 

In the intervening months some lawmakers have expressed frustration over the fact that the Fed has not released all of the material Congress has requested as part of an investigation of the possible leak of information from the central bank to an economic consulting company in 2012. 

Ms Yellen has said the Fed had declined to send the information because a separate Justice Department probe is ongoing.

Her statement was submitted to the committee along with a lengthier report from the Fed board on the state of the economy and financial markets.

That report included more detail on what the US faces as it tries to go its own way in a weakened world economy. 

The expectation that the Fed will diverge from Europe, Japan and other central banks and begin raising rates has pushed up the value of the dollar, and driven down exports and US growth.

This has made the Fed's outlook less certain, the report said.