WASHINGTON — The Federal Reserve launched another aggressive stimulus programme on Thursday, saying it will buy $40 billion of mortgage debt per month and continue to purchase assets until the outlook for jobs improves substantially.

In a significant shift in the direction of US monetary policy, the Fed has tied its unconventional bond buying directly to economic conditions, a move that is likely to be controversial among central bank critics.

“If the outlook for the labour market does not improve substantially, the committee will continue its purchase of agency mortgage-backed securities, undertake additional asset purchases, and employ its other policy tools as appropriate until such improvement is achieved in a context of price stability,” the Fed said in a statement. In an additional step that reflects just how concerned Fed officials have become about the health of the economy, policymakers said they would not likely raise rates from current rock-bottom lows until at least mid-2015. Previously, it had set such guidance at late 2014.

The decision comes in the face of widespread questions about the likely effectiveness of a further foray into unorthodox monetary policy, including from Republican presidential nominee Mitt Romney.

The latest purchases build on the $2.3 trillion in US government and housing-related debt the Fed has already bought.

The new move is even bolder than many investors had anticipated given its open-ended nature and clear links to unemployment.

Wholesale inflation rises

US producer prices rose by the most in three years in August as the cost of energy surged, but underlying inflation pressures were contained, keeping the door open to additional monetary policy easing.

Other data on Thursday underscored the weakness in the labour market, a major concern for the Federal Reserve, with the number of Americans filing new claims for jobless benefits rising to a two-month high, although some of the gain was attributed to Tropical Storm Isaac.

The Labour Department said its seasonally adjusted producer price index increased 1.7 per cent last month, the largest gain since June 2009 and accelerating from July’s 0.3 per cent rise.

Economists polled by Reuters had expected prices at farms, factories and refineries to rise 1.1 per cent last month. Wholesale prices excluding volatile food and energy costs rose 0.2 per cent, slowing from a 0.4 per cent increase in July. The rise matched economists’ expectations.

In a second report, the department said initial claims for state unemployment benefits rose 15,000 to a seasonally adjusted 382,000. Economists polled by Reuters had forecast claims rising to 370,000 last week.

A Labour Department official said Tropical Storm Isaac, which drenched parts of the country, accounted for about 9,000 of the claims filed last week. The number is not adjusted to take normal seasonal patterns into consideration.

Even accounting for the storm, the report suggested little improvement in the labour market after job growth slowed sharply in August.