* Fed to reinvest maturing mortgage debt in Treasuries
* Fed offers more somber assessment of growth
* Dollar seen likely to come under pressure
* Contrast with BOJ inaction may weigh on dollar vs yen
By Masayuki Kitano
TOKYO, Aug 11 (Reuters) - The dollar hovered within sight of a 15-year low versus the yen on Wednesday after the Federal Reserve announced plans to boost a flagging economy by reinvesting money from maturing mortgage bonds in government debt.
The U.S. central bank acknowledged economic growth had slowed in recent months and reiterated its intention to hold benchmark interest rates at record lows for an extended time.
The move marked a policy shift for the Fed, which only a few months ago debated how to start winding down some of its monetary stimulus programmes.
The Fed's decision and its downgraded economic assessment suggest that the central bank could eventually adopt additional monetary stimulus measures, market players said.
"(The Fed's) decision should be seen primarily as a signal to financial markets, consumers and businesses that the FOMC is ready to start a second round of quantitative easing if the Committee sees a double dip around the corner," Rabobank analyst Philip Marey said in a research note, referring to the U.S. central bank's policy-setting Federal Open Market Committee.
The dollar dipped 0.2 percent from late U.S. trading on Tuesday to 85.32 yen JPY=, edging back toward an eight-month low of 85.02 yen hit on trading platform EBS last week.
A further drop below November's low of 84.82 yen would take the dollar to a 15-year low.
"Given the contrast with the BOJ's decision yesterday, we seem to have entered a situation where the yen is likely to rise," said Akira Hoshino, chief manager for the Bank of Tokyo-Mitsubishi UFJ's foreign exchange trading department.
The Bank of Japan kept interest rates steady and held off on new policy steps on Tuesday, saving its limited firepower in case the currency's rise accelerates.
A dollar drop below 84.82 yen could trigger more market speculation about the possibility of Japanese intervention.
But traders think the yen will eventually test those levels as Japan is unlikely to intervene to curb the yen unless dollar/yen falls closer to 80 yen, or its moves become more volatile.
The euro dipped 0.2 percent to $1.3156 EUR=, having pulled back from last week's three-month peak of $1.3334.
But the euro could be poised to rise in the wake of the Fed's decision and economic assessment, analysts said.
"With further policy moves now a clear possibility, the USD will come under pressure beyond the impact of U.S.-Euro area growth divergence alone," analysts at JPMorgan said. (Additional reporting by Wayne Cole in Sydney; Editing by Edmund Klamann)